Add `hard_drive_interface` option for VirtualBox builder

By default, Packer attaches the primary hard disk to an IDE controller.  However, this is sub-optimal as IDE hard disks [do not support asynchronous I/O](http://www.virtualbox.org/manual/ch05.html#iocaching).  This PR introduces a new option, `hard_drive_interface`, that allows customization of the disk controller.  At this time, it only accepts `"sata"`, but could be expanded for more elaborate configurations in the future (e.g., SCSI/SAS).