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.
AFAIK the machine lazy model does not have a `units` attribute. I suppose that's monkey patched somewhere (in machine-view-panel.js?) but we cannot rely on that attribute here because it's not part of the contract. And as a side note, we should also avoid monkey patching stuff like this in general IMHO, but that's not related to your branch.
FYI, the units lazy model list exposes a filterByMachine method that I suppose you can use here instead.