attache_worker was always spelled wrongly.
Use anonymous union (standard feature already used in different
code in SPICE) to have an aliased attached_worker function.
Also removed a parameter from this new callback and deprecate
the old.
Due to C ABI removing a parameter is not an issue,
red-qxl.c:red_qxl_attach_worker will continue to pass the parameter,
new code will ignore, old code will receive it.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>