PCI passthru: tools changes (generic and vt-d)

PCI passthru: tools changes (generic and vt-d)

I have added CONFIG_PASSTHROUGH in ioemu/Makefile.target and
ioemu/hw/pc.c in attached vtd_tools2.patch.  This should turn off
libpci usage by default until user specifically enables it.

This can be safely check-in without breaking builds for people who do
not care about pass-through devices.  I will try to think of a better
way to enable this.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>
Signed-off-by: Guy Zana <guy@neocleus.com>