vnc: threaded VNC server

vnc: threaded VNC server

Implement a threaded VNC server using the producer-consumer model.
The main thread will push encoding jobs (a list a rectangles to update)
in a queue, and the VNC worker thread will consume that queue and send
framebuffer updates to the output buffer.

The threaded VNC server can be enabled with ./configure --enable-vnc-thread.

If you don't want it, just use ./configure --disable-vnc-thread and a syncrhonous
queue of job will be used (which as exactly the same behavior as the old queue).
If you disable the VNC thread, all thread related code will not be built and there will
be no overhead.

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