If the device does not have an Xwayland seat, we would pick the default seat of the screen for cursor warping and confinement.
But there is no guarantee that there is a default seat anyway, with e.g. with weston's headless backend.
So instead of trying the default seat as a fallback, simply ignore requests for devices where a seat is not set.
Signed-off-by: Olivier Fourdan ofourdan@redhat.com