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.
I've updated the comment; lxc's are named based on their parent. E.g. if the parent is "mysql=0", the lxc name is "lxc:mysql=0". This being deployer names, of course.