[Fedora-livecd-list] Making xz compression the default patch

The patch is pretty simple, but I'm also looking for feedback on the change itself. The downside to having it the default (for F15+), is that occassionally someone might get burnt using xz compression while building an iso for an older version of Fedora. I think the upside of using xz is enough that we really should make it the default. (130 MB savings for the Desktop spin.) But I wanted to run this by other people before making that change. Note that Fedora can use xz for its spins (and is already doing so for the nightly composes) without making xz the default compression.