Boot to ISO from Grub.

40_custom

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry "Ubuntu ISO" {
	set root=(hd0,4)
	set iso=/ubuntu.iso
	loopback loop $iso
	linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$iso only-ubiquity
	initrd (loop)/casper/initrd
}