Re-factor pci_add reply parsing and parse domain/bus numbers

Re-factor pci_add reply parsing and parse domain/bus numbers

The current code for parsing pci_add replies ignores the the domain and
bus numbers. Re-write the code to rectify that.

Also, since pci_add is used for NIC hotplug as well ask disk hotplug,
re-factor the code into a separate function.

* src/qemu_driver.c: add qemudParsePciAddReply() function which can
  handle parsing domain and bus numbers