So, if you don't need the network card you can take it out. Or carefully specify in /etc/xen/xend-config.sxp. Or if you just don't want a card to show up you can hide it from dom0. In this case it can actually be used by a guest directly too...

To hide it add the pciback stuff to the dom0 kernel command line, an example follows:

module /boot/vmlinuz-2.6.22-14-xen root=UUID=6af9903c-e2ee-4fe3-86e2-3601d97131a2 ro console=tty0 pciback.permissive pciback.hide=(03:00.0)

If you then want a guest to be able to use that card instead, you can then use something like

pci='03:00.0'

in the guest config file.

I use 03:00.0 since that is the card i wanted to hide. I determined this with the lspci command as below.

# lspci | grep net
03:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5754 Gigabit Ethernet PCI Express (rev 02)
04:02.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8169 Gigabit Ethernet (rev 10)