memory-device: Unify enabled vs. supported error messages

memory-device: Unify enabled vs. supported error messages

Let's unify the error messages, such that we can simply stop allocating
ms->device_memory if the size would be 0 (and there are no memory
devices ever).

The case of "not supported by the machine" should barely pop up either
way: if the machine doesn't support memory devices, it usually doesn't
call the pre_plug handler ...

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-2-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>