SCVMM - DB error VM with multiple VHDs on volume

This PR ensures the VM storages hash does not contain duplicate host volumes. Otherwise a  [ActiveRecord::RecordNotUnique] error will be thrown for the storages_vms_and_templates table.

https://bugzilla.redhat.com/show_bug.cgi?id=1173251
Oh, I think you just want `uniq`, not `uniq!`.  The **bang** will return `nil` if there are no duplicates to delete.

For instance:

```ruby
[1, 2, 3, 3].uniq!    # returns [1, 2, 3]
[1, 2, 3, 3].uniq     # returns [1, 2, 3]
[1, 2, 3].uniq!       # returns nil
[1, 2, 3].uniq        # returns [1, 2, 3]
```