As I'm a Debain user and not aware of Fedora I do not know, but isn't pciback available on every XEN system. As I'm quite new to XEN MythTV and LinuxMCE I cannot give you qualified advises, but have you add pciback.hide(xx:xx.x) to grub menu.lst and pci option to your xen domU config?
As I'm also interested in running LinuxMCE in a domU (i will not do anything else then domU's, no phyical machines anymore), how have you achived the installation? To you have some info's (docu's etc.)?