core: ignore powering-up events when vm is being migrated

No need to decrease the pending memory & cpu because of powering-up
events while the VM is being migrated. The migrate command is
responsible to decrease the pending resources when it is finished.

Change-Id: Ib1c57900fd3fbc0cb9a5953bee90e53ab11ef7c4
Signed-off-by: Arik Hadas <ahadas@redhat.com>