Description:
Update the QoS setting in interface Unsuccessfully by update-device

Versions:
libvirt-3.9.0-2.virtcov.el7.x86_64
qemu-kvm-rhev-2.10.0-6.el7.x86_64

How reproducible:
100%

Steps:
1. Prepare a running guest with xml as below.

# virsh dumpxml rhel | grep /interface -B11
<interface type='network'>
<mac address='52:54:00:90:06:7a'/>
<source network='default'/>
<model type='virtio'/>
<bandwidth>
<inbound average='1000' peak='5000' burst='1024'/>
<outbound average='128' peak='256' burst='256'/>
</bandwidth>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>


2. Check the QoS  rules

#  tc filter show dev vnet0 parent ffff:
filter protocol all pref 49152 u32
filter protocol all pref 49152 u32 fh 800: ht divisor 1
filter protocol all pref 49152 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid :1 not_in_hw
  match 00000000/00000000 at 0
 police 0x1 rate 1024Kbit burst 256Kb mtu 64Kb action drop overhead 0b
        ref 1 bind 1


# tc class show dev vnet0
class htb 1:1 root leaf 2: prio 0 rate 8Mbit ceil 40Mbit burst 1Mb cburst 1600b


3. Update the xml info as below  

# cat interface.xml
<interface type='network'>
<mac address='52:54:00:90:06:7a'/>
<source network='default'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>


# virsh update-device rhel interface.xml
Device updated successfully


4. Check QoS setting of interface

# virsh dumpxml rhel | grep /interface -B9
    <interface type='network'>
      <mac address='52:54:00:90:06:7a'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>



5. Check the QoS  rules
#  tc filter show dev vnet0 parent ffff:
filter protocol all pref 49152 u32
filter protocol all pref 49152 u32 fh 800: ht divisor 1
filter protocol all pref 49152 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid :1 not_in_hw
  match 00000000/00000000 at 0
 police 0x1 rate 1024Kbit burst 256Kb mtu 64Kb action drop overhead 0b
        ref 1 bind 1

# tc class show dev vnet0
class htb 1:1 root leaf 2: prio 0 rate 8Mbit ceil 40Mbit burst 1Mb cburst 1600b



Expected result:
The output should be none
#  tc filter show dev vnet0 parent ffff:
# tc class show dev vnet0


Actual result:
As the step5 shows


Addtional info:
If I delete the interface info and just add the xml to guest xml then start the guest

    <interface type='network'>
      <mac address='52:54:00:90:06:7a'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>


Check the QoS  rules
# tc filter show dev vnet0 parent ffff:
# tc class show dev vnet0