Acceptance Tests: add standard clean up at test tearDown()

The avocado.Test class, used as the basis of the avocado_qemu.Test
class, performs a clean of temporary directories up as part of its own
tearDown() implementation.

But the avocado_qemu.Test class is currently missing the same clean
up, as it implemented its own tearDown() method without resorting to
the upper class behavior.

This brings avocado_qemu.Test behavior in sync with the standard
avocado.Test behavior and prevents temporary directories from
cluttering the test results directory (unless instructed to do so with
Avocado's "--keep-tmp" option).

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
[willianr: respin to new Python super format]
Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210920204932.94132-2-willianr@redhat.com>

tests/acceptance/avocado_qemu/__init__.py

@@ -276,6 +276,7 @@ def tearDown(self):
         for vm in self._vms.values():
             vm.shutdown()
         self._sd = None
+        super().tearDown()
 
     def fetch_asset(self, name,
                     asset_hash=None, algorithm=None,