[Libguestfs] Xen drivers for virt-builder images

I've just been playing around with virt-builder, and it seems like a pretty useful tool. However, it seems that in a number of the images, the initrd file is missing drivers for Xen virtual block and network devices. This results in a VM that can't find its root device. The files needed are drivers/block/xen-blkfront.ko and drivers/net/xen-netfront.ko. They seem to be present in centos-6 and ubuntu-14.04, but not in rhel-7rc or fedora-20. Whom would I contact to get these added? Thanks! -George