uncompressing-.xz: use archive name for .xz files

.xz files extract as original name/data instead of the original name
libarchive returns a single entry called 'data' when raw format is used.
This causes AutoarExtractor to create an unnecessary directory and put
the decompressed file in it.

This patch checks the entry and sets the resulting filename to the
source file.

https://bugzilla.gnome.org/show_bug.cgi?id=777396