control/zone: Use const references when iterating

We don't want to copy the object out of the container we are iterating
over.

Tested:
    Built and run through unit tests.

Change-Id: I68b041f98e1c6a6a9dbc59448b629e1d22ba62c9
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed