Description of problem:
- create vm
- create disk (system, data)
- start vm - vm is started
- stop vm
- add new disk
- edit first disk, unset the bootable checkbox
- edit second disk, set the bootable checkbox
- deactivate first disk
- start vm - vm fails to start
reason: disk boot order sent to vdsm is 2, libvirt reject that operation, as it expect 1 to be sent first ...