1. Update elements or attributes other than "boot order" - PASS(can update);
2. Update "boot order" of interface - PASS(not support);
3. Hotplug with "boot order" with os/boot elements(negative) - PASS(unsupported);
4. Hotplug with duplicate "boot order"(negative) - PASS(report error);
5. Hotplug with "boot order" - PASS;
6. Hotplug without 'boot order' - PASS;
7. Coldplug for 3,4,5,6 - PASS ;
8. update elements or attributes other than "boot order" when vm is inactive - PASS(can update);
9. update "boot order" of interface when vm is inactive - PASS(can update);
10. Invalid "boot order"(negative) when vm is active or inactive - PASS(report error) ;

1. # virsh dumpxml rhel | grep /interface -B9
    <interface type='network'>
      <mac address='52:54:00:0f:96:f6'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <filterref filter='clean-traffic'/>
      <boot order='2'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
modify the nwfilter and add link state
# cat interface1.xml
<interface type='network'>
      <mac address='52:54:00:0f:96:f6'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <filterref filter='allow-ipv4'/>
     <link state='down'/>
      <boot order='2'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
# virsh update-device rhel interface1.xml
Device updated successfully
use dumpxml to check it is updated

2. update the interface boot order from '2' to '3'
# virsh update-device rhel interface2.xml
error: Failed to update device from interface2.xml
error: Operation not supported: cannot modify network device boot index setting

3. update interface to add the <boot order/> with the <boot dev/> defined in vm
# virsh update-device rhel interface4.xml
error: Failed to update device from interface4.xml
error: unsupported configuration: per-device boot elements cannot be used together with os/boot elements

4.  when there is a disk with <boot order='1'/>
# cat interface3.xml
<interface type='network'>
      <source network='default' bridge='virbr0'/>
      <model type='virtio'/>
      <boot order='1'/>
    </interface>
# virsh attach-device rhel interface3.xml
error: Failed to attach device from interface3.xml
error: unsupported configuration: boot order 2 is already used by another device

last comment is tested on libvirt-3.9.0-14.el7_5.4, all the results are as expected.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2018:1396