deltas: Don't try to rollsum/bsdiff .xz files

deltas: Don't try to rollsum/bsdiff .xz files

Fedora switched to 'xz' compress kernel modules, and recently
[RHEL7 did too](https://bugzilla.redhat.com/show_bug.cgi?id=1367496).
This compression defeats bsdiff.

While we have a "rollsum-able" test, we don't have a "bsdiff-able" test as it'd
be very expensive (we'd have to bsdiff, then apply it and compare the result).

Let's do the tactical quick fix here and just not try to delta files ending in
`.xz.`. This avoids us using bsdiff pointlessly for over 4000 files, which is
quite a notable speed increase for generating deltas.

Closes: #1333
Approved by: jlebon