core: on handing over a VM mark it as MigratingTo

This patch aims to reflect the VM handover more clear so instead of
moving a VM to Unknown it will move to MigratingTo (unless the host is
*already* NonResponging) so tracking a VM status transition wouldn't
puzzle users (at least would be less confusing and more script friendly
as the script won't discover a VM is suddenly Uknown)

Non responding treatment has been changed to collect also VMs that are
migrating to the host (i.e status MigratingTo and run_on_vds == nonRespongindVds)

Change-Id: I0cc072b445ef2616e209fd9dfb30f65abc0ad3da
Signed-off-by: Roy Golan <rgolan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/909932