Add an iptables mangle rule per-bridge for DHCP.

When vhost-net is present on a host, and DHCP services are
run on the same system as guests (multi_host), an iptables
rule is needed to fill packet checksums.  This adds a rule
per-bridge for multi_host networks when vhost-net is present,
similar to how newer versions of libvirt handle the issue for
bridges/networks that it manages.

Fixes LP: #1029430

EDIT: Updated tests and pep8.

(Backported from commit 901a3dacb6f2d36cbe8d23707dba75452e91df33)

Change-Id: I1a51c1d808fa47a77e713dbfe384ffad183d6031