Sends Zebra message.

        :param msg: Instance of py:class: `ryu.lib.packet.zebra.ZebraMessage`.
        :return: Serialized msg if succeeded, otherwise None.

def send_msg(self, msg):
        """
        Sends Zebra message.

        :param msg: Instance of py:class: `ryu.lib.packet.zebra.ZebraMessage`.
        :return: Serialized msg if succeeded, otherwise None.
        """
        if not self.is_active:
            self.logger.debug(
                'Cannot send message: Already deactivated: msg=%s', msg)
            return
        elif not self.send_q:
            self.logger.debug(
                'Cannot send message: Send queue does not exist: msg=%s', msg)
            return
        elif self.zserv_ver != msg.version:
            self.logger.debug(
                'Zebra protocol version mismatch:'
                'server_version=%d, msg.version=%d',
                self.zserv_ver, msg.version)
            msg.version = self.zserv_ver  # fixup

        self.send_q.put(msg.serialize())