xenapi: make method signatures match parent class

Fix any inconsistencies in method signatures between the
xenapi driver impl and the ComputeDriver parent class.
Add a test case to prevent future regressions.

Partial-bug: #1333219
Change-Id: Ie0be083c684d557c068f8e69a4f0fc38567a1864
Merge "xenapi: make method signatures match parent class"