VT-d, tools: Intel IGD passthrough 2/2: hvm config file example

VT-d, tools: Intel IGD passthrough 2/2: hvm config file example

Add an option into hvm config file to enable graphics passthrough
including discrete and IGD. To passthrough graphics to guest, need to
set gfx_passthru=1, and also specify graphics device BDF in pci
passthrough option, like pci=['xx:xx.x'] in hvm config file.

Signed-off-by: Weidong Han <weidong.han@intel.com>