Remove xml unit tests for v3 api plugins

Remove a bunch of unit tests for XML support in v3 compute API plugins.
This is another step towarding removing XML support completely from this
API.

Part of blueprint remove-v3-xml-api

Change-Id: Ibc35791cd8593b523ff21d9f604d0263e6da2b0a
Merge "Remove xml unit tests for v3 api plugins"