Well. If you have a look on hook's code than you can make it persistent by removing the code that recreates the storage when VM is started. However than you have to make sure that you either do not relocate VM to another host or purge the local storage image if you do. If you come up with general solution for this or ideas, let me know.

So far we are experimenting with using that local storage for CI slaves. In this case it is not a problem for it not be permanent.