Use scancode instead of keycode names

When running on Xwayland, the keycode mapping property is not available,
which causes unknown keycode mapping errors and the keyboard doesn't
work.

Check for a known scancode (“XK_Page_Up”) which differs to distinguish
between “xfree86” and “evdev” when the there is no keycode name.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>