vnc: Limit r/w access to size of allocated memory This fixes memory reads and writes which exceeded the upper limit of allocated memory vd->guest.ds->data and vd->server->data. Cc: Anthony Liguori <aliguori@us.ibm.com> Signed-off-by: Stefan Weil <weil@mail.berlios.de> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>