Correct MAC representation to match iptables output

Correct MAC representation to match iptables output

We were previously using the netaddr's mac_unix format
(which leaves off leading 0's) to generate iptables rules
based on MAC addresses. While iptables accepts this format,
it's not returned this way in the output so the iptables
rule matching code would never find the match for these
rules, causing the loss of counters on these rules on every
reload.

This patch corrects this with a custom dialect that matches
the iptables format.

Closes-Bug: #1502901
Change-Id: Ia45ebde8c4684e12030469323e18367a54d1518b