Description of problem:
libguestfs has the tar-{in,out} and tgz-{in,out} commands for working with uncompressed and gzip-compressed tarballs.  However, the xz tool (lzma compression) tends to have smaller disk space and comparable or even faster decompression.  Therefore, it would make sense to have txz-{in,out} commands to natively support .xz compression.

Version-Release number of selected component (if applicable):
$ guestfish --version
guestfish 1.0.85

How reproducible:
Always.

Steps to Reproduce:
1. guestfish -x alloc output.img 14M : run : \
  sfdiskM /dev/sda , : \
  mkfs ext2 /dev/sda1 : \
  mount /dev/sda1 / : \
  txz-in something.tar.xz /

Actual results:
txz-in something.tar.xz
txz-in: unknown command

Expected results:
Create a disk image from the .xz-compressed tarball.

Additional info:
Workaround: Manually decompress the tarball, and fall back to tar-in instead of the requested txz-in.