[CentOS] Xen and multiple block devices

Hello,

CentOS 5.0 on x86_64

I'm trying to set up Xen paravirt guests with access to 2 block
devices: One for / and one for swap. The block devices are LVM LVs,
but that shouldn't matter.

When I create the guest using either virt-manager or virt-install, I
get asked which device to use for xvda. But I want more block devices:
xvdb for swap.

I know how to add this as an option to the config file in /etc/xen
'phy:/dev/vg1/guest1-swap,xvdb,w'
But that file only appears AFTER running the installer.

How do I make xvdb visible DURING the install of CentOS?

Thanks,
-- 
Francois Caen