Comment 1 for bug 1730627

Commit c17be3cbfc58e5b54ae1d5ae4714460d7ae2e15c from git://anonscm.debian.org/dpkg/dpkg.git seems to contain a fix.

It can be applied to 1.17.5ubuntu5.7, and I successfully built a package. With said patch applied, xz compressed control.tar files seem to be supported.