Time to actually issue the QMP transactions that create and delete
persistent checkpoints, resolving TODOs intentionally left earlier in
the series.  For create, we only need one transaction: inside, we
visit all disks affected by the checkpoint, and create a new enabled
bitmap, as well as disabling the bitmap of the first ancestor
checkpoint (if any) that also had a bitmap.  For deletion, we need
multiple QMP calls: for each disk, if there is an ancestor checkpoint
with a bitmap, then the bitmap must be merged (including activating
the ancestor bitmap if the leaf node changes), all before deleting the
bitmap from the checkpoint being removed.
Signed-off-by: NEric Blake <eblake@redhat.com>