fix(nas-backup): don't persist unconfirmed bitmap; gate sanity_checks

Two more Copilot review items on PR #13074:

* persistChainMetadata: only store nas.bitmap_name when the agent
  confirms it via BITMAP_CREATED=. Previously fell back to
  decision.bitmapNew if the agent didn't emit it, which could anchor
  the next incremental on a bitmap that doesn't exist (stopped-VM
  pre-seed failure was the trigger Copilot flagged, but the same risk
  applies any time the marker is lost). Leaving it empty correctly
  forces the next backup to a fresh full instead.

* nasbackup.sh: sanity_checks (QEMU >= 4.2 / libvirt >= 7.2) is only
  needed for the incremental backup-begin path. Gate it to
  `OP=backup && -n MODE` so legacy full-only callers and delete /
  stats / rebase operations still work on older host versions.

Author: jmsperu

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupProvider.java

@@ -436,11 +436,14 @@ private List<String> composeParentBackupPaths(Backup parent, long vmId) {
      * other providers can implement their own chain semantics without schema changes.
      */
     private void persistChainMetadata(Backup backup, ChainDecision decision, String bitmapFromAgent) {
-        // Prefer the bitmap name confirmed by the agent (BITMAP_CREATED= line). Fall back to
-        // what we asked it to create — they should match.
-        String bitmap = bitmapFromAgent != null ? bitmapFromAgent : decision.bitmapNew;
-        if (bitmap != null) {
-            backupDetailsDao.persist(new BackupDetailVO(backup.getId(), NASBackupChainKeys.BITMAP_NAME, bitmap, true));
+        // Only persist nas.bitmap_name when the agent confirmed it via BITMAP_CREATED=. If we
+        // fall back to decision.bitmapNew when the agent didn't emit BITMAP_CREATED= (e.g.,
+        // stopped-VM path where the qemu-img pre-seed failed, or running-VM path where libvirt
+        // backup-begin succeeded but the bitmap line wasn't surfaced for any reason), we'd
+        // anchor the next incremental on a bitmap that doesn't exist on the host. Better to
+        // leave it empty so the next backup sees no checkpoint and starts a fresh full chain.
+        if (bitmapFromAgent != null && !bitmapFromAgent.isEmpty()) {
+            backupDetailsDao.persist(new BackupDetailVO(backup.getId(), NASBackupChainKeys.BITMAP_NAME, bitmapFromAgent, true));
         }
         backupDetailsDao.persist(new BackupDetailVO(backup.getId(), NASBackupChainKeys.CHAIN_ID, decision.chainId, true));
         backupDetailsDao.persist(new BackupDetailVO(backup.getId(), NASBackupChainKeys.CHAIN_POSITION,

scripts/vm/hypervisor/kvm/nasbackup.sh

@@ -607,8 +607,13 @@ while [[ $# -gt 0 ]]; do
   esac
 done
 
-# Perform Initial sanity checks
-sanity_checks
+# QEMU >= 4.2 and libvirt >= 7.2 are only required for backup-begin (incremental
+# checkpoints and per-bitmap exports). Legacy full-only backups, plus delete /
+# stats / rebase operations, run on older versions just fine. Gate the version
+# check to the paths that actually need it to preserve backward compatibility.
+if [ "$OP" = "backup" ] && [ -n "$MODE" ]; then
+  sanity_checks
+fi
 
 if [ "$OP" = "backup" ]; then
   STATE=$(virsh -c qemu:///system list | awk -v vm="$VM" '$2 == vm {print $3}')