implement `iso_interface` setting

Adds a new configuration option, `iso_interface`, that can be set to `sata` to mount the ISO on the SATA controller.  Required for OS X, which can then be built by the template at https://github.com/zigg/osx-vm-templates/commit/f7e624dd97274abcf005ea465e2e7fb5e95649a5.

Fixes #822.