ioemu: VNC updates should be sent only when requested.

ioemu: VNC updates should be sent only when requested.

Reading qemu code I realized that the qemu vnc server sometimes sends
framebuffer updates even if the client didn't request any. This is not
consistent with the RFB protocol spec and can break some clients.
This patch strictly enforces compliance with the RFB protocol making
sure framebuffer updates are sent only if the client requested one.
Doing so is more difficult than it seems because some framebuffer
pseudo-encoding updates cannot be discarded but must be sent anyway:
for example desktop resize and pixel format change messages. To solve
the problem I wrote a queue that stores those messages and sends them
as soon as the client asks for an update. Since 90% of the times the
queue is used to store only few elements, the queue allocates 10
elements at the beginning and every time it runs out of elements
allocates other 10 elements. This is should drastically limit the
number of malloc and free needed to maintain the queue. I did some
stress tests in the last couple of days and seems to work well.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>