Using qcow2 with NBD

Filler - need to demonstrate:

  qemu -> (raw) -> qemu-nbd -> (qcow2) -> image.qcow2
  qemu -> (qcow2) -> qemu-nbd -> (raw) -> image.qcow2

maybe a table of pro/con for each

Exposing qcow2 over NBD is too risky without resize, forcing existing documentation to favor raw over NBD, even though this loses out on the other benefits of qcow2.