python.ovs.db.idl: Fix Row.delete() of a row already committed to the db.

python.ovs.db.idl: Fix Row.delete() of a row already committed to the db.

Row.delete() handled the case of deleting a row that was added within the
current transaction, but not yet committed, but it did not correctly handle
the case of deleting a row that belonged to the database before the
transaction started.  This fixes the problem.

Reported-by: Yeming Zhao <zhaoyeming@gmail.com>
Tested-by: Yeming Zhao <zhaoyeming@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>