xenapi: Speeding up more tests by switching to NoDB

nosetests nova/tests/compute/test_compute_xen.py
          nova/tests/virt/xenapi/image/test_bittorrent.py
          nova/tests/virt/xenapi/image/test_glance.py

BEFORE: Ran 13 tests in 3.435s
AFTER: Ran 13 tests in 0.374s

Change-Id: I00cc990dab3f9af2c19aa57b84817fafc1cb01e9
Merge "xenapi: Speeding up more tests by switching to NoDB"