Only can add the cd-rom as PVC not DV in UI

CNV v4.20.0-168

Always

1. Create a VM from InstanceType and customized to add the cd-rom volume before creating the VM
2. Go to UI -> VM -> Configuration -> Storage -> Disks -> CD-ROM disk -> Mount -> Upload ISO
3. Check the volume field in vm yaml 

Only can mount cd rom as PVC 
        - name: cd-rom-aqua-weasel-45
          persistentVolumeClaim:
            claimName: cdrom-volume
          hotpluggable: true

we support both DV and PVC when mount cd-rom to the VM's volume field, better to have a list for user to choose DV or PVC to mount