libvirt: Remove use of db for flavor extra specs in tests

The watchdog tests were using the, now discouraged, approach of calling
db.flavor_extra_specs_update_or_create to set flavour extra specs for
the test case. Rewrite the test cases to mock the Flavor.get_by_id
method to return a fake flavor object.

Change-Id: I92b45762a8f079b0e7bef7f9daf4cf5ada78896f
Merge "libvirt: Remove use of db for flavor extra specs in tests"