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
Updated from global requirements

Change-Id: Icb3c49781f2769698714d4fab9cbaed10ebe4251
Tidy up xenapi/volume_utils.py

Removed unused code and made some private methods start with _

Change-Id: Idae0bf55b9d0586fd0a0412aace517afc46031ea
Move tests into test_volume_utils

Move some tests in test_xenapi that would fit better in
test_volume_utils.

Change-Id: Ibe516a2473ff921d4f43621e71645810e674f73b
VMware: spawn refactor - phase 1 - power_on_vm

move power_on to vm_util and change all power_on in vmops to use it

ensure test_driver_api tests only the driver and not vmops

change id for the last review in this phase 1 series with a spawn
test is Ia137f1ea4b9fa43741cdd7d610831da8e5474d62

partial blueprint vmware-spawn-refactor

Change-Id: I5c1d8bc38c4a7d0107c8d7639e385239e53b070d
VMWare: spawn refactor - phase 1 - create_virtual_disk

Several methods are repeating code for creating the virtual disk.
This refactor isolates the code for that task so it can be reused.

partial blueprint improve-vmware-disk-usage
partial blueprint vmware-spawn-refactor
Co-authored-by: Maithem <munshedm@vmware.com>
Co-authored-by: Tracy Jones <tjones@vmware.com>

Change-Id: If764340be76abbb24e1954f02b3ecb317ba143f9
Merge "Tidy up xenapi/volume_utils.py"
Merge "Move tests into test_volume_utils"
Merge "Updated from global requirements"
Merge "XenAPI: Remove unneeded instance argument from image downloading"
Merge "Add a docstring to hacking unit tests"