docs/zoned-storage:add zoned emulation use case

Add the documentation about the example of using virtio-blk driver
to pass the zoned block devices through to the guest.

Signed-off-by: Sam Li <faithilikerun@gmail.com>
Message-id: 20230508051916.178322-5-faithilikerun@gmail.com
[Fix pre-formatted code syntax
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/devel/zoned-storage.rst

@@ -41,3 +41,22 @@ APIs for zoned storage emulation or testing.
 For example, to test zone_report on a null_blk device using qemu-io is::
 
   $ path/to/qemu-io --image-opts -n driver=host_device,filename=/dev/nullb0 -c "zrp offset nr_zones"
+
+To expose the host's zoned block device through virtio-blk, the command line
+can be (includes the -device parameter)::
+
+  -blockdev node-name=drive0,driver=host_device,filename=/dev/nullb0,cache.direct=on \
+  -device virtio-blk-pci,drive=drive0
+
+Or only use the -drive parameter::
+
+  -driver driver=host_device,file=/dev/nullb0,if=virtio,cache.direct=on
+
+Additionally, QEMU has several ways of supporting zoned storage, including:
+(1) Using virtio-scsi: --device scsi-block allows for the passing through of
+SCSI ZBC devices, enabling the attachment of ZBC or ZAC HDDs to QEMU.
+(2) PCI device pass-through: While NVMe ZNS emulation is available for testing
+purposes, it cannot yet pass through a zoned device from the host. To pass on
+the NVMe ZNS device to the guest, use VFIO PCI pass the entire NVMe PCI adapter
+through to the guest. Likewise, an HDD HBA can be passed on to QEMU all HDDs
+attached to the HBA.