qxl: Remove qxl_bo_init() return value

It's always returning 0, and it's always ignored.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
Message-id: 20161108091209.25568-6-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>