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