async-ify codebase

[danobi]

vmtest is more or less a wrapper around qemu stuff. So it's almost completely
I/O bound, making it a good candidate for async.

Async is particularly appealing cuz we have a bunch of threads in places where
we don't want blocking semantics. And the threads are getting a bit hacky (for
example most of them are never joined and implicitly require the other end of
a socket to hang up for the thread to exit). Aync would let you select
on multiple futures allowing for cleaner cancellation semantics.

Overall I think the codebase would be cleaner with async.