Let user specify a 'base' VM for efficient CoW restore

[tasket]

Differential restore at the VM level

Users may want to restore VMs that are similar to / derived from existing VMs. Util could clone the data of such a base VM before restoring to it, then use Wyng's sparse receive, resulting in efficient space allocation in local storage (as well as more efficient retrieval from the archive).

Implementation:

• Command line option to specify the base VM
• If the base VM is in the restore list, automatically order the restore progression so that the base is restored first