balloon: transitional device support

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.

Three issues with legacy hypervisors have been identified:
1. Actual value is actually used, and is necessary for management
   to work. Luckily 4 byte config space writes are now atomic.
   When using old guests, hypervisors can detect access to the last byte.
   When using old hypervisors, drivers can use atomic 4-byte accesses.
2. Hypervisors actually didn't ignore the stats from the first
   buffer supplied. This means the values there would be
   incorrect until hypervisor resends the request.
   Add a note suggesting hypervisors ignore the 1st buffer.
3. QEMU simply over-writes stats from each buffer it gets.
   Thus if driver supplies a different subset of stats
   on each request, stale values will be there.
   Require drivers to supply the same subset on each
   request. This also gives us a simple way to figure out
   which stats are supported.

VIRTIO-143

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio/branches/v1.0@526 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652

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