core: further handling of unmanaged disk devices

When attempting to restore vm configuration from a snapshot, unmanaged
disk devices can be removed as well and restored from the snapshot if
needed like other unmanaged devices.

Change-Id: Ia0cd3a95de9b04d49cc655981e15abed4f1adab7
Related-To: https://bugzilla.redhat.com/show_bug.cgi?id=1032807
Signed-off-by: Liron Aravot <laravot@redhat.com>