vnc: fix server surface pixel format.

vnc: fix server surface pixel format.

Format must be identical to the guest surface, we can't work with
the 32 bpp used by the default surface allocator.

Without this patch vnc doesn't get the conversions right when sending
pixel data to the client.  The bug triggers if
  (a) the client doesn't support WMVi, and
  (b) the guest screen depth is != 32 bpp.

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