Migration/colo.c: Fix double close bug when occur COLO failover

In migration_incoming_state_destroy(void) will check the mis->to_src_file
to double close the mis->to_src_file when occur COLO failover.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190303145021.2962-2-chen.zhang@intel.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>