Support machine placement in bundle export

* Service placement on machines is based on unit placement, per deployer requirements.
* LXC, though partially implemented, is not officially supported because deployer support is spotty.
`units[0].service` does work; I got stuck on the idea of using ids, clearly.

`owner` is in the sense of who deployer thinks owns the machine, since deployer places units in terms of another service. I've updated the comment to explain this.