create a packet including LACP.

def _create_response(self, datapath, port, req):
        """create a packet including LACP."""
        src = datapath.ports[port].hw_addr
        res_ether = ethernet.ethernet(
            slow.SLOW_PROTOCOL_MULTICAST, src, ether.ETH_TYPE_SLOW)
        res_lacp = self._create_lacp(datapath, port, req)
        res_pkt = packet.Packet()
        res_pkt.add_protocol(res_ether)
        res_pkt.add_protocol(res_lacp)
        res_pkt.serialize()
        return res_pkt