OpenNebula

I propose you to add virtual cdroms support.
As virtual cdroms are use ide bus, we can do this like other hard drives:

In sunstone interface it may look like option when attaching new images.
Along with this options "Image" "Volatile disk", we can add option "CDROM"

In the template config it may look like:

DISK = [
  TYPE = "cdrom" ]

Or like this, if you attached any image:

DISK = [
  TYPE = "cdrom",
  IMAGE = "ubuntu-16.04.2-server-amd64.iso",
  IMAGE_UNAME = "kvaps" ]

Any other settings is same, but need to allow this type without any image setting.

On libvirt level it will look like:

<disk type='file' device='cdrom'>
   <target dev='hdc' bus='ide'/>
   <readonly/>
</disk>

When vm is created, in storage tab it need to have option to attach or detach some iso file into this cdrom.
It may look like two new actions along with "Save as", "Detach", "Snapshot" actions in Sunstone interface.

This actions may use "attach_disk" script from vmm driver for attach needed iso file inplace cdrom or use same script for attach empty xml inplace this CDROM for detaching.