pc: disable pci-info for 1.6

The BIOS that we ship in 1.6 does not use pci info
from host (yet). Several issues turned up
(e.g. around winXP boot crashes). So it's safest to disable that
interface for 1.6 machine types for now, leave it on for 1.7
as we have enough time to fix issues if any.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
