Usage percentage bar is only available for RBD images with "fast-diff" feature enabled (performace reasons).
When an RBD image don't have this required feature, we should hide the usage percentage bar (instead of displaying "0%")
How to reproduce:
- Create an RBD image with "fast-diff" and another without "fast-diff"
- Write to this RBD images (useful tool can be found here - https://github.com/openattic/openattic-vagrant/pull/29)
- Go to RBD datatable and check the "Used" column values