Attach the ISO to an ahci-cd emulated device.  The
ISO will appear to be mounted on a /dev/cd device
instead of /dev/vtbd.  This is similar to how other
virtualization environments handle mounting ISO images.

Reviewed by: neel


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271447 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f