xenbus_directory
int xenbus_directory(struct xenbus_transaction *t,
err = xenbus_directory(NULL, path, "", &orig_dir_n, &dir);
err = xenbus_directory(NULL, "device", "", &dir_n, &dir);
err = xenbus_directory(NULL, "backend", "", &dirt_n, &dirt);
err = xenbus_directory(NULL, "backend", dirt[type],
err = xenbus_directory(t, dir, node, &dir_n, &d);