enable live-restore for PBS

enable live-restore for PBS

Enables live-restore functionality using the 'alloc-track' QEMU driver.
This allows starting a VM immediately when restoring from a PBS
snapshot. The snapshot is mounted into the VM, so it can boot from that,
while guest reads and a 'block-stream' job handle the restore in the
background.

If an error occurs, the VM is deleted and all data written during the
restore is lost.

The VM remains locked during the restore, which automatically prohibits
any modifications to the config while restoring. Some modifications
might potentially be safe, however, this is experimental enough that I
believe this would cause more bad stuff(tm) than actually satisfy any
use cases.

Pool handling is slightly adjusted so the VM can be added to the pool
before the restore starts.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>