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/setup_env.sh

@@ -0,0 +1,91 @@
+#!/bin/sh
+# Prepara o ambiente de VMs: imagens, seed e maquinas virtuais.
+# Requer: libvirt (qemu:///session), virt-install, qemu-img, mkisofs.
+set -e
+
+HERE=$(cd "$(dirname "$0")" && pwd)
+WORK=${WORK:-/var/tmp/vmtest}
+CONN=qemu:///session
+
+mkdir -p "$WORK"
+cd "$WORK"
+
+fetch() {
+    url=$1; out=$2
+    [ -s "$out" ] && { echo "  $out ja existe"; return; }
+    echo "  baixando $out"
+    curl -fsSL -o "$out.part" "$url"
+    mv "$out.part" "$out"
+}
+
+echo "== imagens =="
+fetch https://cloud.debian.org/images/cloud/bookworm/latest/debian-12-genericcloud-amd64.qcow2 debian.qcow2
+fetch https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/cloud/nocloud_alpine-3.20.3-x86_64-bios-cloudinit-r0.qcow2 alpine.qcow2
+
+echo "== redimensionando =="
+for img in debian.qcow2 alpine.qcow2; do
+    size=$(qemu-img info --output=json "$img" | grep -oP '"virtual-size":\s*\K[0-9]+')
+    if [ "${size:-0}" -lt 8589934592 ]; then
+        qemu-img resize "$img" 8G
+    else
+        echo "  $img ja tem 8G"
+    fi
+done
+
+echo "== disco de dados (fontes + script de teste) =="
+if [ ! -s data.img ]; then
+    dd if=/dev/zero of=data.img bs=1M count=8 status=none
+    mkfs.ext4 -q -F data.img
+fi
+
+echo "== seed cloud-init =="
+cat > user-data <<'EOF'
+#cloud-config
+hostname: rfs-test
+ssh_pwauth: true
+disable_root: false
+users:
+  - name: root
+    lock_passwd: false
+    plain_text_passwd: rfstest
+chpasswd:
+  expire: false
+EOF
+cat > meta-data <<'EOF'
+instance-id: rfs-test-1
+local-hostname: rfs-test
+EOF
+cat > network-config <<'EOF'
+version: 2
+ethernets:
+  eth0:
+    dhcp4: true
+EOF
+rm -f seed.iso
+mkisofs -output seed.iso -volid cidata -joliet -rock \
+    user-data meta-data network-config >/dev/null 2>&1
+
+echo "== definindo VMs =="
+define_vm() {
+    name=$1; img=$2; variant=$3; mem=$4
+    if virsh -c "$CONN" dominfo "$name" >/dev/null 2>&1; then
+        echo "  $name ja existe"
+        return
+    fi
+    virt-install -c "$CONN" \
+        --name "$name" \
+        --memory "$mem" --vcpus 2 \
+        --disk "path=$WORK/$img,format=qcow2,bus=virtio" \
+        --disk "path=$WORK/seed.iso,device=cdrom" \
+        --os-variant "$variant" \
+        --network user,model=virtio \
+        --graphics none --console pty \
+        --import --noautoconsole >/dev/null
+    echo "  criada: $name"
+}
+
+define_vm rfs-debian debian.qcow2 debian12 2048
+define_vm rfs-alpine alpine.qcow2 alpinelinux3.20 1024
+
+echo
+echo "Pronto. Use tests/vm/run_in_vm.sh <vm> para testar."