passthrough: Correctly expose PCH ISA bridge for IGD passthrough

passthrough: Correctly expose PCH ISA bridge for IGD passthrough

The i915 driver probes chip version through PCH ISA bridge device / vendor ID.
Previously, the PCH ISA bridge is exposed as PCI-PCI bridge in qemu-xen-trad,
which breaks the assumption of the driver. This change fixes the issue by
correctly exposing the ISA bridge to domU.

Signed-off-by: Rui Guo <firemeteor@users.sourceforge.net>
Tested-by: Rui Guo <firemeteor@users.sourceforge.net>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
(cherry picked from commit 73c4ad5a3d065267b878c8ae31ff0b4833f093e4)