Cleanup virt driver tests and verify final spawn

This removes redundant mocking of the IronicClientWrapper._get_client() method
and instead relies on a local fake client wrapper. This allows us to verify
loopingcall usage in the final stage of instance spawn.

This also moves some client-specific tests out of the driver tests and into
the client wrapper tests where they belong.

Change-Id: I79b05773ff302057af6694bac4951470f8eb7c1a
Merge "Cleanup virt driver tests and verify final spawn"