core : AddVmFromSnapshot - use device information from snapshot.

When adding a vm from snapshot (clone of a snapshot), the device
information was taken from the vm current state and not from the
snapshot configuration - which caused the cloned vm to not be in the
desired state and possibly have missing devices.

Change-Id: I0611bbbef00fd2c2390ead5f3110e56dcaacd579
BUg-Url: https://bugzilla.redhat.com/show_bug.cgi?id=882644
Signed-off-by: Liron Aravot <laravot@redhat.com>