vdpa_sim: get rid of DMA ops

We used to (ab)use the DMA ops for setting up identical mappings in
the IOTLB. This patch tries to get rid of the those unnecessary DMA
ops by maintaining a simple identical/passthrough mappings by
default. When bound to virtio_vdpa driver, DMA API will simply use PA
as the IOVA and we will be all fine. When the vDPA bus tries to setup
customized mapping (e.g when bound to vhost-vDPA), the
identical/passthrough mapping will be removed.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221223060021.28011-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>