Retain PCI address from NIC attach

Retain PCI address from NIC attach

When we pci_add a NIC, we need to retain the PCI address assigned by
qemu for using during detach.

* src/qemu_driver.c: use qemudParsePciAddReply() to pull the PCI
  address from the pci_add reply

* src/domain_conf.c: handle storing and parsing the PCI address in the
  domain state XML file