I am going to guess that you are relying on xen's standard network-bridge script to create your bridges.
I've only used xen as provided by Novell/SuSE, so some of the paths to the files might be different on different distros, but hopefully this will give you the information you need.
By default, xen's network-bridge script will setup a bridge on whichever NIC had the default route. However, you can override this behavior by changing a single line in your /etc/xen/xend-config.sxp file. By default, you'll have a line that looks like this:
You'll want to change it to something like this:
(network-script 'network-bridge netdev=eth0')
assuming eth0 is the interface you want to use for xen networking. That setting will take effect when you reboot your host. However, you can accomplish the same thing without rebooting, by doing the following:
1. Shut down all running VMs.
2. Shut down the bridge by running `/etc/xen/scripts/network-bridge stop`
3. Start the bridge on the correct interface by running `/etc/xen/scripts/network-bridge start netdev=eth0`