Create VM/CT from backup

[rgl]

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Description

In Proxmox we can backup a to a single file as described at https://pve.proxmox.com/wiki/Backup_and_Restore.

As backups are the native way to export/import a VM/CT, it would be nice to have packer use them (like a iso can be used from proxmox-iso builders, but from a backup file url).

Use Case(s)

Create a VM/CT from a backup file.

Potential configuration

from_backup = "http://example.com/debian-12.zst"

Potential References

https://pve.proxmox.com/wiki/Backup_and_Restore