vga: fix region checks in wraparound case

Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20171030102830.4469-1-kraxel@redhat.com
