[nova] Deprecating the XenAPI driver

After discussing this at the Train PTG and logging a quality warning in the driver 3 months ago [1] with no response, the nova team is now formally deprecating the XenAPI driver [2]. There has been no working third party CI for the driver for at least a release and no clear maintainers of the driver in nova anymore.

If you're using the driver in production, please speak up now otherwise technically the driver could be removed as early as the Ussuri release.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006744.html [2] https://review.opendev.org/#/c/680732/