The PCI device addresses are only valid while the VM is running,
since they are auto-assigned by QEMU. After shutdown they must
all be cleared. Future QEMU driver enhancement will allow for
persistent PCI address assignment

* src/conf/domain_conf.h, src/conf/domain_conf.c, src/libvirt_private.syms
  Add virDomainDefClearPCIAddresses() method for wiping out auto assigned
  PCI addresses
* src/qemu/qemu_driver.c: Clear PCI addresses at VM shutdown