virtio_ring: rename dma_handle to map_handle

virtio_ring: rename dma_handle to map_handle

Following patch will introduce virtio map operations which means the
address is not necessarily used for DMA. Let's rename the dma_handle
to map_handle first.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20250821064641.5025-6-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>