Description of problem:
Up until the change that moved the boot property to the relation between a disk and a VM the boot option could not be set when attaching a disk to VM and depended solely on whether the disk is defined as boot or not.
After this change we can decide whether to attach a disk as bootable or not and in that case when we already mark a disk from the list as bootable there's no logic in marking another one as bootable as the operation will fail hence once we mark one the other boxes should be disabled

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

How reproducible:
100%

Steps to Reproduce:
1. Create a VM with either no disks or no bootable disk
2. Attempt to attach another disk from the VM disks subtab -> "Attach"
3. A list of disks will be displayed along with a changeable boot checkboxes for each
4. Mark one of the disks as boot

Actual results:
It is allowed to mark other disks as boot as well

Expected results:
After marking one disk as a boot the other boxes should be disabled

Additional info: