Gets the OpenFlow port number.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get Interface <port> ofport

def get_ofport(self, port_name):
        """
        Gets the OpenFlow port number.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get Interface <port> ofport
        """
        ofport_list = self.db_get_val('Interface', port_name, 'ofport')
        assert len(ofport_list) == 1
        return int(ofport_list[0])