Description of problem:
Deactivating the VM's bootable disk is done without any warning.
This cause the VM to be up (not sure this is OK either) but unusable so at least a warning to the customer about it is expected.  

Version-Release number of selected component (if applicable):
ovirt-engine 4.3.0-0.2.master.20181128141347.git38908dd.el7


How reproducible:
100%

Steps to Reproduce:
1.Create VM with a bootable disk
2.Activate VM
3.Deactivate bootable disk

Actual results:
Deactivating the VM's bootable disk is done without any warning.
This cause the VM to be up (not sure this is OK either) but unusable so a warning to the customer about it is expected.  

Expected results:
A fair warning to the user is expected on this operation like :
"this operation will cause the VM also to be unusable, do you still wish to proceed?"