block: add block_set_io_throttle virtio-blk-pci QMP example

The block_set_io_throttle command can look up BlockBackends by the
attached qdev device ID.  virtio-blk-pci is a special case because the
actual VirtIOBlock device is the "/virtio-backend" child of the PCI
adapter device.

Add a QMP schema example so clients will know how to use
block_set_io_throttle on the virtio-blk-pci device.

The alternative is to implement some sort of aliasing for qmp_get_blk()
but that is likely to cause confusion and could break future use cases.
Let's not go there.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Message-id: 20180117090700.25811-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qapi/block-core.json

@@ -1799,6 +1799,24 @@
 # Example:
 #
 # -> { "execute": "block_set_io_throttle",
+#      "arguments": { "id": "virtio-blk-pci0/virtio-backend",
+#                     "bps": 0,
+#                     "bps_rd": 0,
+#                     "bps_wr": 0,
+#                     "iops": 512,
+#                     "iops_rd": 0,
+#                     "iops_wr": 0,
+#                     "bps_max": 0,
+#                     "bps_rd_max": 0,
+#                     "bps_wr_max": 0,
+#                     "iops_max": 0,
+#                     "iops_rd_max": 0,
+#                     "iops_wr_max": 0,
+#                     "bps_max_length": 0,
+#                     "iops_size": 0 } }
+# <- { "return": {} }
+#
+# -> { "execute": "block_set_io_throttle",
 #      "arguments": { "id": "ide0-1-0",
 #                     "bps": 1000000,
 #                     "bps_rd": 0,