xenapi: Speedup vm_util and vmops tests

* Uses NODBTestCase in more places

* Also, adds VMUtilsTestBase so that all new test cases in test_vm_utils will,
by default, not use the database.

Speed up observed from running:

nosetests nova/tests/virt/xenapi/test_vm_utils.py
          nova/tests/virt/xenapi/test_vmops.py

BEFORE: Ran 113 tests in 7.674s
AFTER: Ran 113 tests in 1.873s

Change-Id: Id05ecac90201fba27cbae7480d3119cacd92ec24
Merge "xenapi: Speedup vm_util and vmops tests"