[libvirt-users] span port from cisco switch to host to guest using kvm

I am trying to get the equivalent of promisc mode to pass all received traffic to a kvm guest that is running snort. So I have a cisco switch with span port plugged into eh2 on my kvm host which is centos. I want the kvm host to forward all promisc traffic from the span port to the guest running snort of tcpdump. What would be the best way to do this?