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

Commit 864ef70 (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.

daemon/tar.c

@@ -405,5 +405,5 @@ int
 do_txz_out (const char *dir)
 {
   optargs_bitmask = GUESTFS_TAR_OUT_COMPRESS_BITMASK;
-  return do_tar_out (dir, "bzip2", 0, NULL);
+  return do_tar_out (dir, "xz", 0, NULL);
 }