vnc: add display id to acl names

In case the display id is "default" (which is the one you get if you
don't explicitly assign one) we keep the old name scheme, without
display, for backward compatibility reasons.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>