tools/libxl/libxl_pci.c: Judge igd through class code instead of device ID

tools/libxl/libxl_pci.c: Judge igd through class code instead of device ID

IGD passthrough couldn't work on Skylake and Kabylake, because their
Device ID aren't in fixup_ids[]. Currently we need to add every intel
graphic ID into fixup_ids[], it is hard to maintain.

This patch judge intel graphics through vendor id (0x8086) and class
code(0x030000), this could support both the old and new intel graphics,
and reduce maintain work in future.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>