Related to discussion #577
When backups are stored on shared storage accessible from multiple nodes,
the backup polling code was incorrectly assigning the backup to whichever
node it was discovered on during the scan, rather than the node where the
VM/container actually resides.
This fix:
- Builds a lookup map of VMID -> actual node at the start of backup polling
- Uses this map to assign the correct node for guest backups (VMID > 0)
- Preserves existing behavior for host backups (VMID == 0)
- Falls back to the queried node if the guest is not found in the map
This ensures the NODE column accurately reflects which node hosts each
guest, matching the information displayed on the main page.
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