Description of problem:
blkiotune cannot live update <weight> value  into domain xml via --weight

Version-Release number of selected component (if applicable):
libvirt-1.2.17-4.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.define and start a guest , get blkiotune as default

# virsh blkiotune r7
weight         : 1000
device_weight  :
device_read_iops_sec:
device_write_iops_sec:
device_read_bytes_sec:
device_write_bytes_sec:

2. set blkiotune with --weight
# virsh blkiotune r7 --weight 999

# virsh blkiotune r7
weight         : 999
device_weight  :
device_read_iops_sec:
device_write_iops_sec:
device_read_bytes_sec:
device_write_bytes_sec:

# virsh dumpxml r7|grep blkiotune
Nothing output .


3. define and start a guest like following
# virsh dumpxml r7-20 | grep blkiotune -A 5
  <blkiotune>
    <weight>888</weight>
    <device>
......

4.set blkiotune with --weight
# virsh blkiotune r7-20 --weight 999

# virsh blkiotune r7-20
weight         : 999
device_weight  :
device_read_iops_sec:
device_write_iops_sec:
device_read_bytes_sec:
device_write_bytes_sec:

# virsh dumpxml r7-20 | grep blkiotune -A 5
  <blkiotune>
    <weight>888</weight>  
    <device>
.....

check in cgroup , the value has been changed.
# cat /sys/fs/cgroup/blkio/machine.slice/machine-qemu\guest-name.scope/blkio.weight
999

Actual results:
As step2 and step4, the value of weight cannot be live updated.

Expected results:
I think the value of <weight> should be live updated.
If it's not (because the default value 1000 is not displayed in domain xml); perhaps It's better give some info in man page or libvirt.org.

Additional info: