Hi,

When using a shared display surface buffer some of the optimizations
done by the vnc server code don't work.  In shared buffer mode the guest
may update the screen while the vnc server looks at the framebuffer.
That in turn makes some code racy, the dirty bitmap walk through for
example, leading to screen corruption.  Right now this is visible with
xenfb only.  I expect simliar issues will show up for vga too once we
run the vcpus in threads.

please apply
  Gerd