blockdev: refactor transaction to use Transaction API

We are going to add more block-graph modifying transaction actions,
and block-graph modifying functions are already based on Transaction
API.

Next, we'll need to separately update permissions after several
graph-modifying actions, and this would be simple with help of
Transaction API.

So, now let's just transform what we have into new-style transaction
actions.

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]>