[nova] Changes for out-of-tree drivers

We're aiming to remove the long-deprecated XenAPI driver before Victoria ends. With that removed, there are a number of XenAPI-specific virt driver APIs that we plan to remove in follow-ups. These are noted at [1]. If you maintain an out-of-tree driver, you will need to account for these changes. Cheers, Stephen [1] https://review.opendev.org/#/c/749300/1/nova/virt/driver.py