source: https://cloud.google.com/compute/docs/instances/enable-nested-virtualization-vm-instances
$ gcloud compute disks create ubuntu-vm-disk --image-project ubuntu-os-cloud --image-family ubuntu-1604-lts --zone us-central1-a
$ gcloud compute images create ubuntu-nested-vm-image \
--source-disk ubuntu-vm-disk --source-disk-zone us-central1-a \
--licenses "https://www.googleapis.com/compute/v1/projects/vm-options/global/licenses/enable-vmx"