Don't do a graceful shutdown before restoring a snapshot

[jandubois]

We are currently shutting down the VM normally by stopping services etc and then shutting down the VM. This only makes sense when we expect to restart the VM again.

When the VM is going to be replaced by the restored snapshot, we could perform the forced shutdown we do for a factory-reset. This should cut the time required for a restore (on filesystems with CoW semantics) almost in half.