Remove straggling use of all-kwarg object methods

This removes one remaining vestige of the days of yore when
nova developers were forced to always use kwargs on object
methods out of fear, lest it be used over RPC.

Related to blueprint unified-object-model

Change-Id: Ieba76efc2f5daed08e4bde9096c0e178f1f44466
Merge "Remove straggling use of all-kwarg object methods"