virtio-scsi: finalize IOMMU support

After converting to use DMA api for virtio devices, we should use
dma_as instead of address_space_memory. Otherwise it won't work if
IOMMU is enabled.

Fixes: commit 8607f5c3 ("virtio: convert to use DMA api")
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <1499170866-9068-1-git-send-email-jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>