Add support for handling exiting of worker thread
Add support for handling exiting of worker thread

All backends currently provide their own implementation for triggering
the worker thread to exit via an EventFd. Modify the VhostUserBackend
trait to allow a common implementation strategy that backends can use to
provide an EventFd (and optional id) that can be used to trigger the
worker to exit.

Signed-off-by: Rob Bradford <c49e1dc14cd52e2644816db47123896e0ac79025@intel.com>

rust-vmm/vhost