Switch egl-headless ui to use qapi DisplayOptions for configuration.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180202111022.19269-8-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>