nasbackup.sh: add timeout, cleanup trap, space check, quiesce support

[jmsperu]

Summary

• Add BACKUP_TIMEOUT (default 6 hours) to prevent indefinitely stuck backup jobs; aborts via domjobabort when exceeded
• Add EXIT trap with cleanup() that resumes paused VMs, removes temp dirs, and unmounts NFS on any exit (error, signal, or normal) — prevents orphan mounts from accumulating
• Add check_free_space() pre-flight check (default 1 GB minimum) before writing backup data
• Add -q/--quiesce flag for optional fsfreeze/thaw via qemu-guest-agent for application-consistent backups
• Use set -eo pipefail for stricter error handling
• Fix mount_operation(): proper if mount instead of broken $? check after pipe
• Quote all variable expansions to prevent word splitting
• Remove manual umount/rmdir from functions (now handled by trap)

Motivation

The current nasbackup.sh has several reliability issues observed in production:

1. No timeout — backup polling loop (until domjobinfo --completed) runs forever if backup stalls, blocking the agent
2. No cleanup on failure — failed backups leave orphan NFS mounts (/tmp/csbackup.XXXXX) and paused VMs that never resume (related: KVM NAS backup: VM remains paused indefinitely when backup job fails #12821)
3. No space check — backups silently fail or produce corrupt output when the NAS is full
4. No quiesce — running VM backups are crash-consistent only, even when qemu-guest-agent is available
5. Broken error handling — $? after a pipe always returns the pipe's exit status, not the command's

Test plan

• Backup a running VM with sufficient space — verify backup completes normally
• Backup a running VM with BACKUP_TIMEOUT=30 — verify timeout and domjobabort
• Backup with NAS at <1GB free — verify pre-flight rejection
• Kill backup mid-run — verify cleanup resumes VM and unmounts NFS
• Backup with -q flag and qemu-guest-agent installed — verify fsfreeze/thaw in logs
• Backup with -q flag without qemu-guest-agent — verify graceful fallback
• Delete backup — verify cleanup trap handles unmount
• Backup a stopped VM — verify space check and error handling on disk convert