tests/acceptance/virtiofs_submounts: use workdir property

tests/acceptance/virtiofs_submounts: use workdir property

For Avocado Instrumented based tests, it's a better idea to just use
the property.  The environment variable is a fall back for tests not
written using that Python API.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reference: https://avocado-framework.readthedocs.io/en/84.0/api/test/avocado.html#avocado.Test.workdir
Message-Id: <20210203172357.1422425-5-crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>