Fix to allow creation of a RAID on HVM instances

This fixes the issue described here:

https://github.com/opscode-cookbooks/aws/issues/80

There's already a similar pull request, but this one allows the user to specify which method they'd like to use.
Is there a good reason to remove this line? If so, it needs to be made clear in the readme that `raid_device_name` is now a required argument