Description of problem:
When editing template's bootable disk (custom template) whose disk source is not dataVolumeTemplate, user can still check the checkbox to "Use this disk as a boot source" in the "Edit disk" modal.

Version-Release number of selected component (if applicable):
4.11.

How reproducible:
always

Steps to Reproduce:
1. Go to VM Template's Disks tab, choose the template with some bootable disk whose disk source is not dataVolumeTemplate
2. Click on "Edit" to edit the disk
3. Look at the checkbox "Use this disk as a boot source"

Actual results:
The checkbox "Use this disk as a boot source" is unchecked, ready to be checked to mark the already bootable disk as bootable.

Expected results:
The checkbox should be already checked and disabled, as the disk is already bootable.

Additional info: