virt: fix signature of set_admin_password method

The compute manager passes (instance, new_pass) to the
set_admin_password method, and the XenAPI driver impl
expects this pair of parameters. The ComputeManager
class, however, declares that it takes (instance, context,
new_pass=None). It makes no sense to default the new
password to None, when the only caller always passes
it, and the context parameter is never passed by the
caller either.

Partial-bug: #1333219
Change-Id: If656da4ef54a0425b9f50ef03af044b803b1afae
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: fix signature of set_admin_password method"
Merge "virt: add test helper for checking public driver API method names"