[Qemu-block] Creating snapshots with specific runtime options

[Alberto Garcia]

As far as I can see there's no way to create a snapshot and either

  a) inherit the runtime options from the original image
  b) specify a new set of options

This comment in external_snapshot_prepare() before calling bdrv_open()
suggests that the problem is known but the discussion was postponed.

    /* TODO Inherit bs->options or only take explicit options with an
     * extended QMP command? */

I would like to retake this and make it possible. I discussed it
briefly with Stefan on IRC and he said that Kevin might have some
ideas.

In principle extending the QMP command sounds as simple as adding
'options': 'BlockdevOptions' to 'blockdev-snapshot-sync', but it's
surely more complicated than that :) Is the 'BlockdevOptions' API even
stable?

Berto