Add host PCI device hotplug support

Add host PCI device hotplug support

Attaching a host PCI device to a qemu guest is done with a
straightforward 'pci_add auto host host=XX:XX.X' command.

Like with NIC and disk hotplug, we need to retain the guest PCI address
assigned by qemu so that we can use it for hot-unplug.

Identifying a device for detach is done using the host PCI address.

Managed mode is handled by detaching/resetting the device before
attaching it to the guest and re-attaching it after detaching it from
the guest.

* src/qemu_driver.c: add qemudDomainAttachHostPciDevice() and
  qemudDomainDetachHostPciDevice()

* src/domain_conf.h: add somewhere to store the guest PCI address

* src/domain_conf.c: handle formatting and parsing the guest PCI
  address