Assign PCI addresses before hotplugging devices

Assign PCI addresses before hotplugging devices

PCI disk, disk controllers, net devices and host devices need to
have PCI addresses assigned before they are hot-plugged

* src/qemu/qemu_conf.c: Add APIs for ensuring a device has an
  address and releasing unused addresses
* src/qemu/qemu_driver.c: Ensure all devices have addresses
  when hotplugging.