When we clone the source VM, it has cloud-init logic which creates/mounts file system on any extra added disks, it can also fill the volumes with data but this takes time. 

Today if the automation code adds disks on the cloned VM, we have to turn on the source VM wait for cloud-init to finish it's job and only then turn off the VM for cold migration, for warm migration keep the vm running but don't start before cloud-init finishes.