xwayland: Do not use a default seat

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