XenAPI: Refactor Fake to create pools, SRs and VIFs automatically

Required for changing the default SR to be the pool default - see
I116f65ece94f839e63b085480ba4f8974119d2bb
This also fixes some minor bugs in fake.py shown up by the tests

Change-Id: I19fc0747648b97a0912d28008981a84f70107d7b
Merge "XenAPI: Refactor Fake to create pools, SRs and VIFs automatically"