add CD-ROM only if not used disk_image, also boot from it only if CD-ROM present

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
disk_filename is used for artifacts, but it missed in case of disk_image = true

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Merge pull request #1662 from vtolstov/disk_image

When using disk_image CD-ROM not needed