Description of problem:

Ovirt offers to delete snapshots for single disks (via storage -> disksnapshots -> select -> delete). This is broken and the engine task does not end. 

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

Ovirt 3.6.6

How reproducible:

100%

Steps to Reproduce:

1. create VM snapshot
2. goto storage -> select storage where VM disks are assigned to (in our case NFS) 
3. Select single disk of vm
4. Remove snapshot


Actual results:

1. Process flow gives two times the completion message.

Disks from snapshot deletion has been completed 
Disks from snapshot deletion has been completed 

2. The deletion task stays active in the engine

Expected results:

1. correct messages
2. Deletion task finishes

Additional info:

screenshots attached