xdg-shell: Fail xdg_surface creation when surface has role

If a given wl_surface already has a role (e.g. cursor or subsurface),
there is nothing you can do with an xdg_surface which won't raise an
error, apart from destroying it.

As of wayland-protocols@11fecf08 this is now explicitly
specified to be illegal, so disallow it within libweston-desktop. This
avoids us tying ourselves in knots with surface-private ownership.

Signed-off-by: Daniel Stone <daniels@collabora.com>