virt: add test helper for checking public driver API method names

The virt driver impls of ComputeDriver should only expose public
methods which are already defined by the ComputeDriver base class.
Any other methods should be prefixed with an '_' to indicate that
they are internal helperrs.

Provide a test helper method which the virt driver tests will
call to validate their implementation is in compliance with
the base class signature.

Partial-bug: #1333219
Change-Id: Ibc2369a5f90e09857da29810ac0707614b041d54
Merge "virt: add test helper for checking public driver API method names"