passthrough: support Intel IGD passthrough with VT-D

passthrough: support Intel IGD passthrough with VT-D

Some registers of Intel IGD are mapped in host bridge, so it needs to
passthrough these registers of physical host bridge to guest because
emulated host bridge in guest doesn't have these mappings.

Some VBIOSs and drivers ssume the IGD BDF (bus:device:function) is
always 00:02.0, so this patch reserves 00:02.0 for assigned IGD in
guest.

(Patch modified slightly by Ian Jackson.)

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>