Comment # 33 on bug 1158817 from Martin Wilck

(In reply to Neil Rickert from comment #28)
> What's "virbr0"?  I don't recall that ever showing up. 

This is the name of the bridge device tha libvirt would typically create. It
actually shows up in the network XML you attached in comment 16.
If you don't see this device, I can only conclude that your "default" network
from libvirt is stopped. How is networking for your VMs set up then? Do you use
static bridges configured with YaST?

> And what does it  mean to "load br_netfilter"? 

br_netfilter is a kernel module, which you an load and unload using "modprobe"
and "rmmod", respectively. The offending directive from comment 2 causes the
br_netfilter module to be loaded whenever the "bridge" module is loaded.
The effect of loading this module is that netfilter hooks (in other words:
firewall rules) are applied for network packets passing bridges, which wouldn't
happen otherwise.