If domain got removed during the tests (for example DispVM), vm.close()
wouldn't be called in cleanup and some file descriptors will be
leaked. Add event handler for cleaning this up. Do not use close()
method here, because it is destructive, but the object may still be used
by the test.
- Add missing loop.run_until_complete() calls.
- Convert subprocess.Popen to asyncio.create_subprocess_exec where
needed (when called process needs to communicate with qubesd).
- Cleanup processes (call .wait()).
Changed the inheritance hierarchy:
1. Renamed `SystemTestsMixin` to `SystemTestCase`
2. `SystemTestCase` is a child of `QubesTestCase`
3. All classes extending the prior `SystemTestsMixin` now just extend `object`
