CentOS virt - Oct 2010 - Best way to give guest promiscuous interface?

I am using CentOS 5.5 with KVM as a host and have a need for one guest
to be able to have promiscuous read access to one port on the physical
host (which is an HP BL460 G6 blade server with Intel 2 port 1Gb
ethernet mezzanine card.)

I initially tried to use PCI passthrough, but I was unable to get it
to work. I set everything up as documented in RHEL documentation
(http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Virtualization/chap-Virtualization-PCI_passthrough.html,
but I got the following error message:
# virsh start c7e1b6_g1
error: Failed to start domain c7e1b6_g1
error: this function is not supported by the hypervisor: Device
0000:0b:00.0 is behind a switch lacking ACS and cannot be assigned


Does anyone have PCI passthrough working on a BL460 G6 with CentOS 5.5?

I then tried to connect the guest to a bridge.  This worked once I did
a "brctl setageingtime 0" on the bridge.
However, I don't know the best way to make this change persistent.
Any suggestions?

Is there a better way to do this?

Thanks for any input,
   David