tests/avocado: machine aarch64: standardize location and RO access

tests/avocado: machine aarch64: standardize location and RO access

The tests under machine_aarch64_virt.py and machine_aarch64_sbsaref.py
should not be writing to the ISO files.  By adding "media=cdrom" the
"ro" is automatically set.

While at it, let's use a single code style and hash for the ISO url.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-ID: <20240806173119.582857-5-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240830133841.142644-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>