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>

tools/examples/xmexample.hvm

@@ -344,6 +344,12 @@ tsc_mode=0
 # 
 #pci_power_mgmt=0
 
+#   Enable graphics passthrough:
+#
+#   If it's set, and specify grapchis device BDF in pci passthrough option,
+# like pci=['xx:xx.x'], it enables graphics passthrough, default=0 (disabled)
+#gfx_passthru=0
+
 #-----------------------------------------------------------------------------
 #   Configure PVSCSI devices:
 #