avocado_qemu: standardize super() call following PEP3135

PEP3135 states when calling super(), there is no need to use arguments.
This changes the calls on avocado_qemu to standardize according to
PEP3135 and avoid warnings from linters.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210920204932.94132-3-willianr@redhat.com>