Code review comment for lp:~cbehrens/nova/lp844160-build-works-with-zones

You'll see some code removed from the volumes extension. This is because it subclasses the servers controller to override create() to allow 'block_device_mapping' to be specified with a build. Since it was mostly code duplication (and used create_instance_helper), I just created a method in the servers controller that can be overridden in the volumes extension to retrieve the block_device_mapping.