Fix launch errors, improve Pods API, use hook abstraction

Sorry for the largeness of this PR, just took a while to get it all together. @mpuncel @petertseng 

This extracts the preparer's orchestration logic to a package that can be tested. It does not solve the self-hosting issue yet, but gets us most of the way there.
It's really annoying when the VM halts after a failed test because the first thing I want to do is get on the VM to debug. This leaves it on in case of failure.