virtio: introduce virtio_map container union

virtio: introduce virtio_map container union

Following patch will introduce the mapping operations for virtio
device. In order to achieve this, besides the dma device, virtio core
needs to support a transport or device specific mapping metadata as well.
So this patch introduces a union container of a dma device. The idea
is the allow the transport layer to pass device specific mapping
metadata which will be used as a parameter for the virtio mapping
operations. For the transport or device that is using DMA, dma device
is still being used.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20250821064641.5025-5-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>