virtio_ring: skip unmap for premapped

Now we add a case where we skip dma unmap, the vq->premapped is true.

We can't just rely on use_dma_api to determine whether to skip the dma
operation. For convenience, I introduced the "do_unmap". By default, it
is the same as use_dma_api. If the driver is configured with premapped,
then do_unmap is false.

So as long as do_unmap is false, for addr of desc, we should skip dma
unmap operation.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Message-Id: <20230810123057.43407-7-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>