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