With one owner per VM, the VM page and a user's page counted the VM's
owners and then added one in separate steps, so two owners added at the
same moment both found the VM without one, and it got two. The check
and the insert now run in one transaction that holds the ownership lock
(PostgreSQL's advisory lock; SQLite's IMMEDIATE transactions already
serialize them): the second waits for the first and is refused. The
audit row is written after the transaction, so a failure keeps its row.
