Fixed utilization trend data (Max Used/Avg Used) for datastore

The following methods -
```
:v_derived_storage_used
:min_v_derived_storage_used
:max_v_derived_storage_used
```

that populate the trending data for datastore were not being called earlier due to the type of accessor specified.
Changed the accessor type from ```attr_accessor``` to ```attr_accessible```

https://bugzilla.redhat.com/show_bug.cgi?id=1089625