vnc_jpeg and vnc_png are now "auto" by default, this means that
if the dependencies are installed (libjpeg or libpng), then they
will be enabled.

vnc_thread is disabled by default. It should be enabled by default
as soon as it's stable enougth.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>