/** * Register a watch on the given path/path2, using the given xenbus_watch * structure for storage, and the given callback function as the callback. * Return 0 on success, or -errno on error. On success, the watched path * (path/path2) will be saved as watch->node, and becomes the caller's to * kfree(). On error, watch->node will be NULL, so the caller has nothing to * free, the device will switch to XenbusStateClosing, and the error will be * saved in the store. */