Make unit: and index: standard across docs and examples

[maggie44]

The examples read (https://github.com/balena-labs-research/balenaVirt/blob/master/examples/guests.aarch64.yml):

      - if: virtio
        format: qcow2
        unit: 0
        file: /data/{{templateName}}{{guestId}}.qcow2

The docs read:

drive:
  - if: virtio
    format: qcow2
    file: /data/guest{{guestId}}.qcow2
    index: 0
    media: disk

Caused a bit of a pause while working out whether index: or unit: were necessary. We could standardise the docs and examples to use the same.