txz-out: Fix this API to produce xz files again (RHBZ#972775).

Commit 864ef706a84c7c87a62f776d4b2e82a6f959ae14 (in Aug 2012) broke
the deprecated txz-out API.  Because of a thinko during this commit,
instead of producing xz files it produces bzip2 files.

Fix this API again.

Thanks: Bo Fan / Red Hat QA team.