VT-d: constrain IGD check

VT-d: constrain IGD check

Marking a DRHD as controlling an IGD isn't very sensible without
checking that at the very least it's a graphics device that lives at
0000:00:02.0. Re-use the reading of the class-code to control both the
clearing of "gfx_only" and the setting of "igd_drhd_address".

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
master commit: f8c4317295fa1cde1a81779b7e362651c084efb8
master date: 2023-03-14 10:44:08 +0100