xz: support concatenated .xz streams

function                                             old     new   delta
xz_dec_reset                                           -      77     +77
unpack_xz_stream                                    2402    2397      -5

Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>