virtio-gpu: fix ui idx check

Fix off-by-one value check (0 is the first scanout).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1463653560-26958-7-git-send-email-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
