QEMU processes started by Acceptance Tests are left running

Every now and then, QEMU processes started by the Acceptance Tests (thus by Avocado) will be left running.

From Avocado's perspective, when everything "goes well" and a test reaches completion, there's no attempt to terminate any processes it indirectly started.  Some frameworks and tests built on top of Avocado, for instance Avocado-VT, will keep processes running between various tests.

When a job (and consequently a test) is manually interrupted, then Avocado tries to terminate the entire process tree. 

It may be possible to improve the situation in which, at the very least, the user is:
 * notified of left over processes
 * have a configuration option that will attempt to kill all processes at the end of the test execution