Description of problem:
When adding a disk by "Custom path", input a disk with backing file firstly, then select "Device" to "CD/DVD disc", the error is still there, but the "Add" button is enabled.

Version-Release number of selected components (if applicable):
cockpit-284-1.el8.x86_64
cockpit-machines-284-1.el8.noarch
libvirt-dbus-1.3.0-2.module+el8.8.0+16781+9f4724c2.x86_64

How reproducible:
100%

Steps to Reproduce:
1 Create a closed VM, then enter the VM detail page
2 Click "Add", then select "Source" to "Custom path"
3 Input a disk with backing file
4 Select "Device" to "CD/DVD disc"

Actual results:
1 After step 3, there is an error shown, the "Add" button is disabled
2 After step 4, the error is still there, but the "Add" button is enabled

Expected results:
1 After step 3, there is an error shown, the "Add" button is disabled
2 After step 4, the error is still there, the "Add" button is also disabled

Additional info: