ui: ignore hardware keycode 255 on win32

It is a reserved value and doesn't have a corresponding
valid scancode.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20180117164717.15855-5-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
