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.

fsck/semantic_check.c

@@ -551,7 +551,10 @@ static int check_semantic_pass(struct reiserfs_key *key,
 
 		for (i = pos_in_item; i < get_ih_entry_count(&tmp_ih);
 		     i++, deh++) {
-			struct item_head relocated_ih;
+			/* Only meaningful when check_check_regular_file() returns
+			   RELOCATED; that path currently passes NULL for new_ih,
+			   so this stays zero-initialised. */
+			struct item_head relocated_ih = { 0 };
 			int ret = OK;
 
 			if (name) {