Stops uBridge.

def _stop_ubridge(self):
        """
        Stops uBridge.
        """

        if self._ubridge_hypervisor and self._ubridge_hypervisor.is_running():
            log.info("Stopping uBridge hypervisor {}:{}".format(self._ubridge_hypervisor.host, self._ubridge_hypervisor.port))
            yield from self._ubridge_hypervisor.stop()
        self._ubridge_hypervisor = None