xenapi: Speeding up the easy cases of test_xenapi

These were the tests that only required changing the base class. The remaining
tests in test_xenapi will need to have their stubs modified to not hit the DB.

nosetests nova/tests/virt/xenapi/test_xenapi.py

BEFORE: Ran 201 tests in 25.957s
AFTER: Ran 201 tests in 24.573s

Change-Id: Ia0478ac81f1b2b316a22b15e3171250d3b839073
Merge "xenapi: Speeding up the easy cases of test_xenapi"