tests/avocado/machine_aarch64_sbsaref.py: allow for rw usage of image

tests/avocado/machine_aarch64_sbsaref.py: allow for rw usage of image

When the OpenBSD based tests are run in parallel, the previously
single instance of the image would become corrupt.  Let's give each
test its own snapshot.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20240806173119.582857-9-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-ID: <20240830133841.142644-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>