vm: Bring back missing before_device_migrate_source hook call

During the merge of libvirtvm.py and vm.py this section of the code
somehow got removed, this patch brings it back.

Change-Id: I317ae6b0ed5a7a6fa0f959169b27339c4451b35a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1048763
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23006
Reviewed-by: Dan Kenigsberg <danken@redhat.com>