Add a libvirt/KVM test environment and fix the gcc-12 warning it found

The VM harness (tests/vm/) builds and exercises the tools on real Debian and
Alpine guests, driving the serial console (no sshd/cloud-init dependency).

Results on Debian 12 (glibc 2.36, gcc 12.2): 12/12 checks pass, 0 warnings
with -Wall -Wextra, including a real 128M -> 256M resize and the rollback
save/restore/reject paths.

Results on Alpine 3.20 (musl 1.2.5): the build cannot start. configure stops
at "reiserfsprogs requires a method to add printf modifiers" because
reiserfscore/prints.c depends on glibc's register_printf_specifier(3) and
<printf.h>, which musl does not provide. This is pre-existing and out of
scope here; measured at 341 use sites across 20+ files, all routed through
reiserfs_warning()/reiserfs_info(). Documented in tests/vm/RESULTS.md.

Fix the one real defect the harness surfaced:

fsck/semantic_check.c: check_check_regular_file() is called with NULL for
new_ih (line 466), so the block that would fill *new_ih is unreachable and
check_semantic_pass() never returns RELOCATED through that path. The
"case RELOCATED:" arm still reads relocated_ih.ih_key, which gcc 12.2
reports as "may be used uninitialized" (-Wmaybe-uninitialized); gcc 16 does
not, because it proves the path dead. Zero-initialise the struct and say why.

Verified: 0 warnings on gcc 12.2 after the change, and the Debian battery
stays at 12/12.

tests/vm/guestcmd.py

@@ -0,0 +1,126 @@
+#!/usr/bin/env python3
+"""Roda um comando no guest atraves do console serial do libvirt.
+
+O console serial e o unico canal garantido nas imagens cloud minimas: nao
+depende de sshd, de cloud-init nem de rede. Este script sincroniza com o
+prompt, autentica (root/rfstest) e executa o comando, lendo a saida do pty
+em blocos pequenos para evitar a sobreposicao de eco que ocorre em comandos
+longos.
+
+Uso:
+    guestcmd.py <dominio> '<comando>'
+    guestcmd.py <dominio> -f <arquivo>       # executa um script local
+"""
+import base64
+import os
+import re
+import select
+import subprocess
+import sys
+import time
+
+USER = "root"
+PASSWORD = "rfstest"
+
+
+def console_pty(domain):
+    xml = subprocess.run(["virsh", "dumpxml", domain],
+                         capture_output=True, text=True).stdout
+    m = (re.search(r"<console[^>]*>\s*<source path='([^']+)'", xml)
+         or re.search(r"<serial[^>]*>\s*<source path='([^']+)'", xml))
+    if not m:
+        sys.exit(f"console serial nao encontrada para {domain}")
+    return m.group(1)
+
+
+class Console:
+    def __init__(self, path):
+        self.fd = os.open(path, os.O_RDWR | os.O_NOCTTY)
+
+    def send(self, s):
+        os.write(self.fd, s.encode())
+
+    def drain(self, seconds):
+        buf = b""
+        end = time.time() + seconds
+        while time.time() < end:
+            r, _, _ = select.select([self.fd], [], [], 0.3)
+            if not r:
+                continue
+            try:
+                chunk = os.read(self.fd, 65536)
+            except OSError:
+                break
+            if not chunk:
+                break
+            buf += chunk
+        return buf
+
+    def login(self):
+        self.send("\n")
+        time.sleep(1.0)
+        snap = self.drain(2.5).decode(errors="replace").lower()
+        if "login:" not in snap:
+            return          # ja autenticado
+        self.send(USER + "\n")
+        self.drain(3.0)
+        self.send(PASSWORD + "\n")
+        time.sleep(2.0)
+        self.drain(2.0)
+
+    def run(self, command, timeout=120.0):
+        """Executa e devolve a saida, sincronizando por marcador no pty."""
+        out_path = "/tmp/.guestcmd.out"
+        self.send(f"rm -f {out_path}; ( {command} ) > {out_path} 2>&1; "
+                  f"echo __GC_DONE__\n")
+        deadline = time.time() + timeout
+        while time.time() < deadline:
+            if b"__GC_DONE__" in self.drain(2.0):
+                break
+        else:
+            return "(timeout)\n"
+
+        # tamanho em bytes
+        self.send(f"wc -c < {out_path}\n")
+        time.sleep(1.2)
+        raw = self.drain(2.0).decode(errors="replace")
+        nums = re.findall(r"\d+", raw)
+        size = int(nums[-1]) if nums else 0
+
+        # ler em blocos base64 (evita eco/sobreposicao)
+        data = b""
+        while len(data) < size:
+            self.send(f"dd if={out_path} bs=1 skip={len(data)} count=150 "
+                      f"2>/dev/null | base64 -w0; echo\n")
+            time.sleep(0.8)
+            enc = self.drain(2.0).decode(errors="replace")
+            token = "".join(re.findall(r"[A-Za-z0-9+/=]{4,}", enc))
+            if not token:
+                break
+            try:
+                data += base64.b64decode(token + "=" * (-len(token) % 4))
+            except Exception:
+                break
+        return data.decode(errors="replace")
+
+
+def main():
+    if len(sys.argv) < 3:
+        sys.exit(__doc__)
+    domain = sys.argv[1]
+    console = Console(console_pty(domain))
+    console.login()
+
+    if sys.argv[2] == "-f":
+        with open(sys.argv[3]) as fh:
+            script = fh.read().replace("'", "'\\''")
+        command = f"sh -c '{script}'"
+    else:
+        command = sys.argv[2]
+
+    sys.stdout.write(console.run(command))
+    sys.stdout.write("\n")
+
+
+if __name__ == "__main__":
+    main()