implement vnc_dpy_setdata

The comment is wrong, we have to do something in the setdata callback.
Changing the framebuffer backing storage (happens when the guest pans
the display) renders the whole screen content invalid.

Trigger #1: cirrus vga + 32bit linux guest + vesafb with ypan enabled.
Trigger #2: std vga + http://patchwork.ozlabs.org/patch/145479/

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
