Instead of relying on QEMU to assign PCI addresses and then querying
them with 'info pci', manually assign all PCI addresses before starting
the guest.  These addresses are not stable across reboots. That will
come in a later patch

NB, the PIIX3 (IDE, FDC, ISA-Bridge) will always have slot 1 and
VGA will always have slot 2. We declare the Virtio Balloon gets
slot 3, and then all remaining slots are for configured devices.

* src/qemu/qemu_conf.c: If -device is supported, then assign all PCI
  addresses when building the command line
* src/qemu/qemu_driver.c: Don't query monitor for PCI addresses if
  they have already been assigned
* tests/qemuxml2argvdata/qemuxml2argv-hostdev-pci-address-device.args,
  tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args,
  tests/qemuxml2argvdata/qemuxml2argv-sound-device.args,
  tests/qemuxml2argvdata/qemuxml2argv-watchdog-device.args: Update
  to include PCI slot/bus information