blockdev: allow BdrvActionOps->commit() to be NULL

blockdev: allow BdrvActionOps->commit() to be NULL

Some QMP 'transaction' types don't need to do anything on .commit().
Make .commit() optional just like .abort().

The "drive-backup" action will take advantage of this, it only needs to
cancel the block job on .abort().  Other block job actions will probably
follow the same pattern, so allow .commit() to be NULL.

Suggested-by: Eric Blake <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>