PacketIn event handler. when the received packet was LACP,
        proceed it. otherwise, send a event.

def packet_in_handler(self, evt):
        """PacketIn event handler. when the received packet was LACP,
        proceed it. otherwise, send a event."""
        req_pkt = packet.Packet(evt.msg.data)
        if slow.lacp in req_pkt:
            (req_lacp, ) = req_pkt.get_protocols(slow.lacp)
            (req_eth, ) = req_pkt.get_protocols(ethernet.ethernet)
            self._do_lacp(req_lacp, req_eth.src, evt.msg)
        else:
            self.send_event_to_observers(EventPacketIn(evt.msg))