Set daemon for IO thread

The IO thread is meant to be a background task.  Mark it
as such to be treated properly on interpreter exit.

Change-Id: I73003625d014f823d26c55bdd22217e19f6bf1c9

pyghmi/ipmi/private/session.py

@@ -391,6 +391,7 @@ class Session(object):
             iothreadwaiters.append(initevt)
             _IOWorker = define_worker()
             iothread = _IOWorker()
+            iothread.daemon = True
             iothread.start()
             initevt.wait()
         elif not iothreadready: