Jcc hvm raid fix

Somewhat primitive but effective fix for ebs_raid that moves from creating devices named sdX1, sdX2, ... to instead creating devices named sdf, sdg, sdh, etc.  This is needed to support HVM images (still works with PV images).  This fix addresses this ticket: https://tickets.opscode.com/browse/COOK-4600.

I call it primitive because it would be nice to have an option allowing the use of either naming style on demand.  It's a start though.