Implement new balloon device (ID 13)

The current virtio balloon device is bad enough that fixing it yields something completely incompatible:
1) The page size is 4096, always, rather than some configurable page size.
2) Page numbers are 32 bits.
3) The deflate queue is not actually used by any device implementation (see issue virtio-25).
4) The "actual" number is kind of useless, and (along with the deflate queue) requires the guest to track pages, rather than just faulting them in.
5) The config is little-endian by mistake (may not matter).

We could just move the entire thing to legacy, and have a new balloon device, like:
1) An incoming queue for balloon size updates or stats request.
2) An outgoing queue for giving 64 bit address/length pairs to balloon, answering stats.
3) No config space, no features.

Thoughts?