xs_register_watch
err = xs_register_watch(&target_watch);
error = xs_register_watch(&xbb->hotplug_watch);
xs_register_watch(&xctrl->xctrl_watch);
error = xs_register_watch(&watch->watch);
xs_register_watch(&ivars->xd_otherend_watch);
xs_register_watch(&ivars->xd_local_watch);
xs_register_watch(&xbs->xbs_device_watch);
xs_register_watch(&ivars->xd_otherend_watch);
int xs_register_watch(struct xs_watch *watch);