make-release: switch to .xz format by default

make-release: switch to .xz format by default

For a long time, we provide two compression formats in the
download area, .bz2 and .xz.  There's absolutely no reason
to provide two in parallel, .xz compresses better, and all
the links we use points to .xz.  Downstream distributions
mostly use .xz too.

For the release maintenance providing two formats is definitely
extra burden too.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 9bc9e95119445d7a430b0fc8b7daf22a3612bbd3)