Troubles doing transparent proxy for virtual machines

[Dan Higgins <dhiggins@ngenera.com>]

Hi iptables gurus. First here is the basic topology:

   Internet
      |
   Gateway
      |
Workstation---eth0---virbr0
                        |
                        +-----+-----+
                        |     |     |
                       vm1   vm2   vm3

I need to test a traffic analyzer running on my workstation, listening
on some port (say 8990) on eth0.

This is not my code, I'm just the poor guy who has to test it. 
Philosophically,
I don't even like what it does, but pragmatically, I got bills to pay.

The rule [I think] I want is:
"Any packets leaving virbr0 going anywhere to port 80 must instead go to 
port
8990 on eth0 (10.0.0.10)."
The software running on port 8990 does its own check of the NAT packet 
mangling
to push the packets through after it inspects them.

I've been banging my head on this, with different variants of:
iptables -t nat -A PREROUTING -i virbr0 -p tcp --dport 80 -j DNAT \
     --to 10.0.0.10:8990
And I've tried the more generic method of using the mangle table with
--set-mark and ip rule add fwmark, but I'm not getting it.

I guess what's confusing me is that everything runs on the same box.

Thanks for any guidance,
Dan