This is needed to make 'cdrom0' device unavailable during the ISO installation process.
It is required for some guest OS types, such as FreeBSD and OmniOS.
CD/DVD drive with installation ISO should be exactly the 2nd device in the VM boot order.
This ensures that VM will boot from this ISO at the first boot time only.
For each ISO image the individual cdrom device will be added to the VM. During the cleanup these devices will be deleted.
It makes attach steps more clear - there is no doubt what is the name of the device.
Related to: [mitchellh/packer#1502]
Mikhail Zholobov
Rickard von Essen
Mitchell Hashimoto