If15ea8b628a6f88211a5d5cc7aadff44f7840138 introduced some mistakes that
I was able to identify when rebasing RequestSpec object patch:
* Missing loading volume_types relationship in QualityOfServiceSpecs
_from_db_object.
* Wrong source of qos_specs relationship in VolumeType _from_db_object.
* Inefficient loading od QoSSpecs associations in db layer.
* Use of self in classmethod.
This commit fixes the issues.
Change-Id: I3b78127791a03e1a35a30d7f84f66e97d89c7cf9
Closes-Bug: 1603472