Fix a regression in xmlGetDocCompressMode()

The switch to xzlib had for consequence that the compression
level of the input was not gathered anymore in ctxt->input->buf,
then the parser compression flags was left to -1 and propagated
to the resulting document.
Fix the I/O layer to get compression detection in xzlib,
then carry it in the input buffer and the resulting document

  This should fix
    https://lsbbugs.linuxfoundation.org/show_bug.cgi?id=3456