Volume management WIP

First pass on volume management API with an initial ZFS implementation.

Note: this is not at all final and not targeting master.  Some interfaces are not yet complete, and some are in fact just documentation indicating the direction they're expected to go.


I think as I've implemented it so far, I've only used it for a pool name.  But I'm pretty sure you're right, it's just the base position in the hierarchy.  I'll rename that when I add tests for that behavior.