OnApp_VirtualMachine_BillingStatistics::_tag_root incorrect

I don't know if there is a difference in versions below 3.0, however, in the API for 3.0 and above, when calling `OnApp_VirtualMachine_BillingStatistics->getList($vmID);`, no data is placed in each array item. 

When the tag_root is changed to `vm_stats` instead of `vm_stat` the data is correctly populated.

I can't find where historical information for the API is, so for backwards compatibility adding `$this->_tagRoot = 'vm_stats` after line 111 in the same file may be a better solution.