fix(instances): two owners added at once keep a VM to one owner

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.

computes/utils.py

@@ -53,7 +53,8 @@ OWNERSHIP_LOCK = 0x7776635F6F776E
 def lock_ownership():
     """Hold the ownership step of every compute until this transaction ends:
     a bury and a restore of the same UUID on two computes must not see each
-    other's work half done. SQLite's IMMEDIATE transactions already do this."""
+    other's work half done, nor two owners added at once each other's check.
+    SQLite's IMMEDIATE transactions already do this."""
     if connection.vendor == "postgresql":
         with connection.cursor() as cursor:
             cursor.execute("SELECT pg_advisory_xact_lock(%s)", [OWNERSHIP_LOCK])