network: don't add iptables rules for externally managed networks

network: don't add iptables rules for externally managed networks

This patch addresses https://bugzilla.redhat.com/show_bug.cgi?id=760442

When a network has any forward type other than route, nat or none, the
network configuration should be done completely external to libvirt -
libvirt only uses these types to allow configuring guests in a manner
that isn't tied to a specific host (all the host-specific information,
in particular interface names, port profile data, and bandwidth
configuration is in the network definition, and the guest
configuration only references it).

Due to a bug in the bridge network driver, libvirt was adding iptables
rules for networks with forward type='bridge' etc. any time libvirtd
was restarted while one of these networks was active.

This patch eliminates that error by only "reloading" iptables rules if
forward type is route, nat, or none.