libvirt: make method signatures match parent class

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

Partial-bug: #1333219
Change-Id: I221433273bd73bb84ed62c8579d45471ac65e1ef
Merge "libvirt: make method signatures match parent class"