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