balloon: allow transitional devices

Virtio 1.0 cs02 doesn't include a modern balloon device. At some
point we'll likely define an incompatible interface with a
different ID and different semantics. But for now, it's not a
big effort to support a transitional balloon device: this has the
advantage of supporting existing drivers, transparently, as well
as transports that don't allow mixing virtio 0 and virtio 1
devices. And balloon is an easy device to test, so it's also
useful for people to test virtio core handling of transitional
devices.