exploiting the new interface in vnc.c (Stefano Stabellini)

exploiting the new interface in vnc.c (Stefano Stabellini)

This patch exploits the new DisplaySurface and PixelFormat structures in
vnc, making the code easier to read allowing further improvements.

Compared to the last version I fixed a bug that prevented the hextile
encoding from working properly.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
As adapted by Stefano to qemu-xen-unstable in his posting:
 [Xen-devel] [PATCH 5 of 13] exploiting the new interface in vnc.c