Leverage mount option for quick VM restoration on KVM/Proxmox

[deajan]

As the title suggests, including the cube/proxmox plugins would allow to use the mount feature in order to "boot from backup".
Not sure if reasonably feasable, since we would have a RO copy, on which we would need to add a VFS layer, and merge that layer at the end of restoration.

This would make a nice killer feature.