network: delay global firewall setup if no networks are running

network: delay global firewall setup if no networks are running

Creating firewall rules for the virtual networks causes the kernel to
load the conntrack module. This imposes a significant performance
penalty on Linux network traffic. Thus we want to only take that hit if
we actually have virtual networks running.

We need to create global firewall rules during startup in order to
"upgrade" rules for any running networks created by older libvirt.
If no running networks are present though, we can safely delay setup
until the time we actually start a network.

Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>