core: monitoring - don't handle devices for MigratingTo VMs

There is no point of trying to update devices of migrating VM. This
could wait when the VM will be up. its consistent with the same
behaviour of ignoring
the state of VMs with status MigratingTo.

Change-Id: I33087d86c4f41d781331d1879297350d3483728f
Signed-off-by: Roy Golan <rgolan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/922490