Replace

   -balloon virtio

With

   -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

This allows it to get correct assigned PCI address as declared in
previous patch

 * src/qemu/qemu_conf.c: Convert Virtio ballon to -device and
   give it an explicit PCI address
 * tests/qemuxml2argvdata/qemuxml2argv-*args: Add in virtio balloon
   where appropriate