Input: xen - check return value of xenbus_printf

Internally, xenbus_printf uses memory allocation, so it can fail under
memory pressure, leaving the input device configured as absolute with the
backend supplying relative coordinates.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>