Add hacking test to block cross-virt driver code usage

The baremetal driver has been accessing some libvirt driver
private classes and config variables. Any code that is intended
to be shared across virt drivers should be in a common package
instead. Add a hacking file to enforce this for imports and
config options so that no further problems like this are made
in the future.

NB, this skips verification of the baremetal driver until
bugs 1261826 and 1261827 are fixed, or the baremetal driver is
removed, whichever comes first.

Change-Id: Ifbfe597795795a847830f9bd937dc459cd37d118
Closes-Bug: #1261842
Add XenAPI driver deprecation warning log message

The XenAPI driver does not have running CI and per the schedule,
is slated for deprecation in Icehouse-3. This adds the warning
message to the log to alert admins to this fact, in line with the
others we have been adding for drivers in similiar situations.

Change-Id: Iaff48aaca27d87c600291b8abcc23273a879366a
Merge "Add hacking test to block cross-virt driver code usage"