Maybe a better way to do this would be after deployment is complete, from an independent box. I believe VCD may require vm to be powered off for adding disks or it may require a reboot for the guest os to see the new disk.
I'm not sure of any way to identify the VM to VCD APIs from within the VM. You may also need to know the org, org vdc etc to properly navigate VCD APIs.
HTH.
Thanks Rags
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