apt-utils: xz support is gone.

Stable release update justification:

[Impact] apt-ftparchive fails to produce .xz compressed files with sufficiently large data sets. This prevents migration of the Ubuntu archive to .xz.
[Test Case] Grab trusty/main/amd64 with debmirror, construct a suitable file list, and ask apt-ftparchive to generate files for this with Packages::Compress set to include xz. We can easily test this on the Launchpad "dogfood" instance where we do QA of the archive publisher.
[Regression Potential] Confined to lzma/xz compression, which I think is basically just apt-ftparchive, but we should make sure that other compression methods still work there.

Original report follows:

Imported from Debian bug http://bugs.debian.org/751688:

Package: apt-utils
Version: 0.9.15.5
Severity: normal

Dear Maintainer,

Tested with 1.0.4 and reinstalled working 0.9.15.5

Packages::Compress seems to not work anymore with xz compression.

Only . (uncompressed), gzip and bzip2 seems to work, xz now return errors
(see at the bottom).

Same error with Sources::Compress

Christian

$ apt-ftparchive generate apt.conf
 dists/unstable/main/binary-amd64/:E: write, still have 32768 to write but
couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 32768 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)
E: write, still have 4096 to write but couldn't
E: IO to subprocess/file failed - write (0: Success)

E: Sub-process Compress child returned an error code (100)