Adds a NIO as new port on Ethernet switch.

        :param nio: NIO instance to add
        :param port_number: port to allocate for the NIO

def add_nio(self, nio, port_number):
        """
        Adds a NIO as new port on Ethernet switch.

        :param nio: NIO instance to add
        :param port_number: port to allocate for the NIO
        """

        if port_number in self._nios:
            raise DynamipsError("Port {} isn't free".format(port_number))

        yield from self._hypervisor.send('ethsw add_nio "{name}" {nio}'.format(name=self._name, nio=nio))

        log.info('Ethernet switch "{name}" [{id}]: NIO {nio} bound to port {port}'.format(name=self._name,
                                                                                          id=self._id,
                                                                                          nio=nio,
                                                                                          port=port_number))
        self._nios[port_number] = nio
        for port_settings in self._ports:
            if port_settings["port_number"] == port_number:
                yield from self.set_port_settings(port_number, port_settings)
                break