core: clone VM

Implementation of the clone VM feature (in CloneVmCommand).

The implementation is done by extracting the common logic from
AddVmFromSnapshotCommand to AddVmAndCloneImageCommand and using it in the
CloneVmCommand.

Change-Id: I3038638314e399e9d2390e7204ce4cc00e85e6ae
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1058832
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>