qxl: reset update_surface

update init_qxl_ram to reset update_surface to 0. This fixes one case
of breakage when installing an old driver in a vm that had a new driver
installed. The newer driver would know about surface creation and would
change update_surface to !=0, then a reset would happen, all surfaces
are destroyed, then the old driver is initialized and issues an
UPDATE_AREA, and spice server aborts on invalid surface.

RHBZ: 690427

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
