Description of problem:
When snapshots are merged rather extensive amount of storage space is needed.
It would be adequate to implement a warning dialogue which would calculate the space needed for merge and display that information to users of RHEV Manager so that they'd be aware of this.

Version-Release number of selected component (if applicable):
3.4

How reproducible:
100 %

Steps to Reproduce:
1. Merge of snapshot tries to allocate (Original image size + snapshot size) on top of already allocated space in order to cope with the merge. 

It would be nice to have that displayed directly to users.

Actual results:
Merge may fail when there's not enough space on the storage domain

Expected results:
User can review if their current environment is able to handle the merge
or if it needs to be extended with more storage space instead of being led to failure of the merge.