switch balloon initialization to -device.

switch balloon initialization to -device.

With that patch applied "-balloon virtio,args" becomes a shortcut for
"-device virtio-balloon-pci,args".

Side effects:
 - ballon device gains support for id=<tag>.
 - ballon device is off by default now.
 - initialization order changes, which may in different pci slot
   assignment depending on the VM configuration.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>