virt: move assertPublicAPISignatures into base test class

The nova/tests/virt/test_driver.py has a useful method for
validating subclass APIs vs the base class contract. This
can be used in many places in Nova, so move it into the
base TestCase class for broader reuse.

Related-bug: #1369151
Change-Id: Ib49e888aae31fb0d47c1d11f35648e9cb2595c75
Merge "virt: move assertPublicAPISignatures into base test class"