virtio_balloon: transitional interface

Virtio 1.0 doesn't include a modern balloon device.
But it's not a big change to support a transitional
balloon device: this has the advantage of supporting
existing drivers, transparently.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>