Don't show warnings on ADMIN_down instance failover

Before:
$ gnt-instance failover -f inst1
… checking disk consistency between source and target
… - WARNING: Can't find disk on node node21.example.com
… shutting down instance on source node

After:
$ gnt-instance failover -f inst1
… not checking disk consistency as instance is not running
… shutting down instance on source node

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