[nova] FYI to out-of-tree drivers: adding a positional argument to finish_revert_migration()

[1] will most likely merge in its current shape or something very close to it. It adds a `migration` positional argument to the finish_revert_migration() virt driver method. `migration` is the, err, migration that's being reverted. If you have out of tree virt drivers, please update them to match this new method signature. Thanks! [1] https://review.opendev.org/#/c/668631/