avocado.utils.archive: Add support for .tar.xz

This patch wraps LZMA archive in order to work with python 2.7 tarfile.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #254 from ldoktor/tar

avocado.utils.archive: Add support for .tar.xz [V3]