Description of problem:
Delete the Qos settings for the interface on the fly makes no changes, as the traffic control rules still exists.

Version-Release number of selected component (if applicable):
libvirt-3.2.0-5.el7.x86_64
qemu-kvm-rhev-2.9.0-5.el7.x86_64

How reproducible:
100%

Steps to Reproduce:

1. bandwidth set in interface
# virsh dumpxml rhel7 | grep /interface -B11
    <interface type='network'>
      <mac address='52:54:00:1e:42:96'/>
      <source network='default' bridge='virbr0'/>
      <bandwidth>
        <inbound average='1000' peak='5000' burst='1024'/>
        <outbound average='128' peak='256' burst='256'/>
      </bandwidth>
      <target dev='vnet0'/>
      <model type='rtl8139'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

# virsh net-dumpxml default
<network connections='1'>
  <name>default</name>
  <uuid>ec95afa8-2529-4e3f-b98c-82cfde363cfd</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:82:0b:86'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.100'/>
    </dhcp>
  </ip>
</network>

2. 
#  tc class show dev vnet0
class htb 1:1 root leaf 2: prio 0 rate 8000Kbit ceil 40000Kbit burst 1Mb cburst 1600b

#  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
  match 00000000/00000000 at 0
 police 0xf rate 1024Kbit burst 256Kb mtu 64Kb action drop overhead 0b
ref 1 bind 1

3. delete the Qos settings by update-device, the command succeed.
# cat nobound.xml
  <interface type='network'>
      <mac address='52:54:00:1e:42:96'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='rtl8139'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

# virsh update-device rhel7 nobound.xml
Device updated successfully

# virsh dumpxml rhel7 | grep /interface -B7
    <interface type='network'>
      <mac address='52:54:00:1e:42:96'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='rtl8139'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

4. Even the command succeed, the rules still exists, and the bandwidth limitation still works.
#  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
  match 00000000/00000000 at 0
 police 0xf 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 8000Kbit ceil 40000Kbit burst 1Mb cburst 1600b

Actual results:
in step 4, all the rules still exists.

Expected results:
in step 4, all the rules should be deleted as there are no traffic control.

Additional info:
When there is no Qos settings for the interface, add Qos settings by update-device works.