[nova] Quality warning patch for xenapi driver

As discussed during today's nova meeting [1] I've posted a change [2] to the xenapi driver such that it logs a warning on startup saying its not tested (no 3rd party CI) and as such may be subject to deprecation in the future. This is kind of the first step in formally deprecating the driver. If you have a vested interest in maintaining the xenapi driver please speak up here and/or on the change.

[1] http://eavesdrop.openstack.org/meetings/nova/2019/nova.2019-05-30-14.00.log.... [2] https://review.opendev.org/#/c/662295/