Use an enum for the vnc feature bits.  That way they are enumerated
automatically and we don't have to do that manually when adding or
removing features.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201208115737.18581-4-kraxel@redhat.com