def _interface_to_service(iface):
    '''
    returns the coresponding service to given interface if exists, otherwise return None
    '''
    for _service in _get_services():
        service_info = pyconnman.ConnService(os.path.join(SERVICE_PATH, _service))
        if service_info.get_property('Ethernet')['Interface'] == iface:
            return _service
    return None
returns the coresponding service to given interface if exists, otherwise return None