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: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20180117090700.25811-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>