[Libguestfs] RFC: async plugin support

I've created a workable async framework with the latest nbdkit code and a sample file plugin using aio to service requests. This allows for a single worker thread to issue multiple requests in parallel via a callback mechanism. Existing locking mechanisms are preserved and if an async thread model sets THREAD_MODEL < PARALLEL then nbdkit continues to properly serialize. The changes are split into 3 commits: 1) backend plumbing changes required to keep the memory for requests around until completed 2) the async plugin backend 3) a sample plugin that uses aio and the async plugin backend The patches would be large so rather than post them all here I've included a link to the github diff https://github.com/libguestfs/nbdkit/compare/master...dev-cloudbd:async