Add a docstring to hacking unit tests

This commit adds a docstring to the hacking check unit tests
explaining how the tests work. To someone who isn't well versed with
how pep8/flake8 works the tests cases are a bit of black magic.

Change-Id: I5dc61612a5fec42443d8d89eaf700bb17b4b0842
XenAPI: Remove unneeded instance argument from image downloading

The Xenapi image downloading code requires an instance argument
when launching a VM.  However, the downloading code doesn't
need the instance information as it doesn't use it for
any processing/debugging/loging.  We can remove this
argument safely.

Change-Id: Id8b531b74e5f0e59638b608bff93fc89b66b889d
Closes-bug: #1307662
Merge "XenAPI: Remove unneeded instance argument from image downloading"
Merge "Add a docstring to hacking unit tests"