This allows libvirt to open the PCI device sysfs config file prior
to dropping privileges so qemu can access the full config space.
Without this, a de-privileged qemu can only access the first 64
bytes of config space.

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Detect support
  for pci-assign.configfd option. Use this option when formatting
  PCI device string if possible
* src/qemu/qemu_driver.c: Pre-open PCI sysfs config file and pass
  to QEMU