get_bus_id
bus_id = get_bus_id(emm_switch);
set_bus_id(&emm_dma, get_bus_id(rsp_sts));
int (*get_bus_id)(char bus_id[XEN_BUS_ID_SIZE], const char *nodename);
err = bus->get_bus_id(devname, xendev->nodename);
.get_bus_id = backend_bus_id,
.get_bus_id = frontend_bus_id,