Description of problem:
live update interface attributes to null report succeed but no changes

Version-Release number of selected component (if applicable):
libvirt-4.10.0-1.module+el8+2317+367e35b5.x86_64

How reproducible:
100%

Steps to Reproduce:

Summary:
update <rom enabled='no'/> to NULL, report succeed but no changes
update <boot order='1'/> to NULL, report succeed but no changes
same rom file, rom bar

1. Start a guest with interface as below:
# virsh dumpxml rhel | grep /interface -B8
    <interface type='direct'>
      <mac address='52:54:00:78:36:26'/>
      <source dev='em1' mode='bridge'/>
      <target dev='macvtap0'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <rom enabled='yes' bar='on' file='/usr/share/ipxe/1af41000.rom'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>

2. dump the interface xml and modify the xml to be without rom setting, then update:
# cat interface.xml
    <interface type='direct'>
      <mac address='52:54:00:78:36:26'/>
      <source dev='em1' mode='bridge'/>
      <target dev='macvtap0'/>
      <model type='virtio'/>
      <alias name='net0'/>
    </interface>

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

3. check the rom setting still there:
# virsh dumpxml rhel | grep /interface -B8
    <interface type='direct'>
      <mac address='52:54:00:78:36:26'/>
      <source dev='em1' mode='bridge'/>
      <target dev='macvtap0'/>
      <model type='virtio'/>
      <alias name='net0'/>
   ** <rom enabled='yes' bar='on' file='/usr/share/ipxe/1af41000.rom'/> **
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>

Actual results:
live update rom settings of interface to be null report succeed but no changes in xml

Expected results:
Report error when live update rom 'enabled' attribute like:
"error: Operation not supported: cannot modify network rom xxx"

Additional info: