memory-device: Introduce machine_memory_devices_init()

memory-device: Introduce machine_memory_devices_init()

Let's intrduce a new helper that we will use to replace existing memory
device setup code during machine initialization. We'll enforce that the
size has to be > 0.

Once all machines were converted, we'll only allocate ms->device_memory
if the size > 0.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-3-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>