[Machines] Uncheck default disk won't disable the arrows when changing boot order

Version-Release number of selected components (if applicable):
cockpit-machines-294-1.el9.noarch
cockpit-296-1.el9.x86_64
libvirt-dbus-1.4.1-5.el9.x86_6
libvirt-client-9.3.0-2.el9.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a shutoff vm.

2. Click "edit" after "Boot order", there will be 2 options listed and the arrows for moving the items are disabled as gray

3. Check the box before "network"  to enable the arrows

4. Uncheck the box before default disk (Note that this is the first raw on the dialog, uncheck other raws would work fine.)

The arrows are not disabled as gray which should be.

5. Click one of  the arrow to change the boot order

Order is changed and the arrows are gray now. 



Actual results:
See step 4.



Expected results:
Uncheck no matter the first or other raws should have the same 



Additional info:
Uncheck other raws except the first raw works fine.