cmdlib: Handle locking.ALL_SET correctly when copying locks

When locks are copied “locking.ALL_SET” must be handled separately
(ALL_SET has the value None). Reported by Constantinos Venetsanopoulos
who saw failover for RDB-based instances not working.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>