[libvirt] [PATCH 3/3] Implement non-pool-blocking volume allocation for FS volumes

Implement non-pool-blocking volume allocation for FS volumes. We setup the volume object, then drop the pool lock, and start the actual storage allocation. Certain pool operations will be blocked (start, stop, destroy, refresh), but the basic things like volume listings will still work. This also allows storage allocation progress reporting: the API user can watch the volume object in a separate thread, polling it's 'info' for update to date capacity/allocation reporting. - Cole