There are now several possible orders of events between sending a 'kill'
and receiving something that indicated a properly shut down inferior.
Coordinating the InferiorExited state during engine ramp down is not
worthwhile as that state had only a forced transition to InferiorShutdownOk
anyway. For the user (and user code), only the fact that ramp down is
finished is interesting.

Change-Id: Ic22131d5a1066dc62bd069f532c28b773f231088
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>