[nova] FYI for out-of-tree virt drivers : new argument for finish_migration() and finish_revert_migration()

I wrote two changes [1] for passing down target allocations to virt drivers given we need to use them for closing some bugfix [2] Given the Nova contributor policy is saying that the public API is not related to virt drivers, we won't support virt drivers that won't accept this new argument. If you're a contributor that works on any out-of-tree Nova virt driver, please make sure that you modify your own two above methods. Thanks, -Sylvain [1] https://review.opendev.org/#/c/589085/7 and https://review.opendev.org/#/c/712118/2 [2] https://bugs.launchpad.net/nova/+bug/1778563