blockdev: abort transactions in reverse order

blockdev: abort transactions in reverse order

Presently, we abort transactions in the same order they were processed in.
Bitmap commands, though, attempt to restore backup data structures on abort.

That's not valid, they need to be aborted in reverse chronological order.

Replace the QSIMPLEQ data structure with a QTAILQ one, so we can iterate
in reverse for the abort phase of the transaction.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20181221093529[email protected]>
[eblake: rebase]
Signed-off-by: Eric Blake <[email protected]>