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/run_in_vm.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+# Envia o codigo-fonte e o script de teste para dentro de uma VM e executa.
+#
+# Uso: run_in_vm.sh <dominio>
+# Requer: guestcmd.py (mesmo diretorio), mkisofs, libvirt.
+set -e
+
+DOMAIN=${1:?uso: run_in_vm.sh <dominio>}
+HERE=$(cd "$(dirname "$0")" && pwd)
+REPO=$(cd "$HERE/../.." && pwd)
+WORK=${WORK:-/var/tmp/vmtest}
+
+echo "== empacotando o codigo-fonte =="
+tar czf "$WORK/rfs-src.tar.gz" -C "$REPO" \
+    --exclude=.git --exclude='*.o' --exclude='*.lo' --exclude='*.la' \
+    --exclude='.libs' --exclude=build-aux --exclude=m4 \
+    --exclude=autom4te.cache --exclude=config.h --exclude=config.log \
+    --exclude=config.status --exclude=libtool \
+    --exclude='Makefile' --exclude='Makefile.in' --exclude=configure \
+    --exclude=aclocal.m4 --exclude='*.8' \
+    .
+
+echo "== montando o disco de dados =="
+rm -rf /tmp/vmdata.$$ && mkdir -p /tmp/vmdata.$$
+sudo mount -o loop "$WORK/data.img" /tmp/vmdata.$$
+sudo cp "$WORK/rfs-src.tar.gz" "$HERE/run_tests.sh" /tmp/vmdata.$$
+sudo umount /tmp/vmdata.$$ && rmdir /tmp/vmdata.$$
+
+echo "== transferindo para $DOMAIN =="
+ATTACH=/tmp/vdb.$$.xml
+cat > "$ATTACH" <<EOF
+<disk type='file' device='disk'>
+  <driver name='qemu' type='raw'/>
+  <source file='$WORK/data.img'/>
+  <target dev='vdb' bus='virtio'/>
+</disk>
+EOF
+virsh attach-device "$DOMAIN" "$ATTACH" --live >/dev/null 2>&1 || true
+rm -f "$ATTACH"
+
+echo "== extraindo e rodando os testes =="
+"$HERE/guestcmd.py" "$DOMAIN" '
+set -e
+mkdir -p /mnt/data
+mountpoint -q /mnt/data || mount -o ro /dev/vdb /mnt/data
+mkdir -p /root/rfs
+tar xzf /mnt/data/rfs-src.tar.gz -C /root/rfs
+cp /mnt/data/run_tests.sh /root/run_tests.sh
+chmod +x /root/run_tests.sh
+SRC=/root/rfs sh /root/run_tests.sh
+' 900