clearly mark DMA_OPS support as an architecture feasture

[Christoph Hellwig]

On Mon, Aug 26, 2024 at 02:27:27PM +0800, Jason Wang wrote:
> Actually I meant, we can extend the virtio_config_ops to allow mapping
> ops there, then simulator and VDUSE can hook the map ops there.

>From a quick glance that feels like the right layer of abstraction,
although the config part of the name feels wrong at that point.