PLXCOMP-239 - Add support for XZ compression and XZ compressed tarballs

PLXCOMP-239 - Add support for XZ compression and XZ compressed tarballs
following the same pattern as BZip2 support.

Also reduce duplication of code from TarCompressionMethod in TarArchiver
and TarUnArchiver

Signed-off-by: Mat Booth <mat.booth@redhat.com>