KVM NAS backup: resume VM and exit on backup failure

[jmsperu]

Summary

Fix three bugs in nasbackup.sh that caused VMs to remain paused indefinitely when backup jobs fail (e.g. storage full, I/O error):

1. Infinite polling loop: backup_running_vm() falls through the Failed case without exiting, causing the script to poll the already-failed job forever. Fixed by adding exit 1 after cleanup.

2. Continued processing after failure: backup_stopped_vm() continues processing subsequent disks after qemu-img convert fails. Fixed by adding exit 1 after cleanup.

3. VM never resumed: cleanup() removes temp files and unmounts but never resumes the VM that was paused by virsh backup-begin. Fixed by adding a VM state check and virsh resume at the top of cleanup().

Root Cause

When virsh backup-begin starts a backup, the VM may be paused. If the backup fails for any reason (storage full, network issue, I/O error), the script's cleanup path never calls virsh resume, leaving the VM paused until manual intervention.

Test Plan

• Trigger a backup with insufficient NAS storage — verify VM is resumed after failure
• Trigger a backup with NAS unmounted mid-backup — verify VM is resumed
• Normal backup completes successfully (no regression)
• Verify qemu-img convert failure on stopped VM exits cleanly

Fixes #12821

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 3.70%. Comparing base (e93ae1a) to head (8550e5a).

❗ There is a different number of reports uploaded between BASE (e93ae1a) and HEAD (8550e5a). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (e93ae1a)	HEAD (8550e5a)
unittests	1	0

Additional details and impacted files

@@              Coverage Diff              @@
##               4.22   #12872       +/-   ##
=============================================
- Coverage     17.61%    3.70%   -13.91%     
=============================================
  Files          5917      448     -5469     
  Lines        531430    38028   -493402     
  Branches      64973     7036    -57937     
=============================================
- Hits          93586     1409    -92177     
+ Misses       427288    36432   -390856     
+ Partials      10556      187    -10369     

Flag	Coverage Δ
uitests	3.70% <ø> (ø)
unittests	?

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