Let's avoid iterating over all devices and simply track it in the
DeviceMemoryState.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-11-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>