What were you trying to do that didn't work?
Update disk with diskcard_no_unref should fail with a clear error message

Please provide the package NVR for which bug is seen:
libvirt-10.3.0-1.el9.x86_64
qemu-kvm-9.0.0-1.el9.x86_64

How reproducible:
100%

Steps to reproduce
1. Prepare a running guest with the following disk.

1. virsh dumpxml rhel --xpath //disk
   <disk type="file" device="disk">
     <driver name="qemu" type="qcow2"/>
     <source file="/var/lib/libvirt/images/rhel.qcow2" index="2"/>
     <backingStore/>
     <target dev="vda" bus="virtio"/>
     <alias name="virtio-disk0"/>
     <address type="pci" domain="0x0000" bus="0x04" slot="0x00" function="0x0"/>
   </disk>
   <disk type="file" device="disk">
     <driver name="qemu" type="qcow2"/>
     <source file="/var/lib/libvirt/images/test.qcow2" index="1"/>
     <backingStore/>
     <target dev="vdb" bus="virtio"/>
     <alias name="virtio-disk1"/>
     <address type="pci" domain="0x0000" bus="0x08" slot="0x00" function="0x0"/>
   </disk>
   2. Prepare vdb disk with discard_no_unref attribute.
2. cat disk.xml
   <disk type="file" device="disk">
     <driver name="qemu" type="qcow2" discard_no_unref="on"/>
     <source file="/var/lib/libvirt/images/test.qcow2"/>
     <target dev="vdb" bus="virtio"/>
     <alias name="virtio-disk1"/>
     <address type="pci" domain="0x0000" bus="0x08" slot="0x00" function="0x0"/>
   </disk>
   3. Update-device with the above disk xml.
3. virsh update-device rhel disk.xml
   Device updated successfully
   4. Check the disk xml but no discard_no_unref attribute.
4. virsh dumpxml rhel --xpath //disk
   ......
   <disk type="file" device="disk">
     <driver name="qemu" type="qcow2"/>
     <source file="/var/lib/libvirt/images/test.qcow2" index="1"/>
     <backingStore/>
     <target dev="vdb" bus="virtio"/>
     <alias name="virtio-disk1"/>
     <address type="pci" domain="0x0000" bus="0x08" slot="0x00" function="0x0"/>
   </disk>

Expected results
Update disk device with discard_no_unref attribute failed, just like discard attribute:

1. virsh update-device rhel disk.xml
   error: Failed to update device from disk.xml
   error: Operation not supported: cannot modify field 'discard_no_unref' of the disk

Actual results
Update disk device with discard_no_unref attribute successfully but it didn't actually work.