blockdev: qmp_transaction: drop extra generic layer

Let's simplify things:

First, actions generally don't need access to common BlkActionState
structure. The only exclusion are backup actions that need
block_job_txn.

Next, for transaction actions of Transaction API is more native to
allocated state structure in the action itself.

So, do the following transformation:

1. Let all actions be represented by a function with corresponding
   structure as arguments.

2. Instead of array-map marshaller, let's make a function, that calls
   corresponding action directly.

3. BlkActionOps and BlkActionState structures become unused

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>