express,fedora: Don't use remote repos for F16

Now that F17 has been out for a while and F18 isn't that far off, F16
isn't important enough anymore to justify (very slow) installation from
network and the synchronous network availability check. Better just not
install buggy vdagent/QXL.

https://bugzilla.gnome.org/show_bug.cgi?id=680825