Removed attr_accessor in order for the methods to be called

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 attr_accessor being specified.

https://bugzilla.redhat.com/show_bug.cgi?id=1089625
Merge pull request #788 from AparnaKarve/bz1089625_fix_utilization_trend_data_for_datastore

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