I found guestfs_compress_out do the compression to gzip, bzip2,
compress...etc.
I want to compress qcow2, and I found details about qcow2 compression in
https://people.gnome.org/~markmc/qcow-image-format.html
The QCOW format supports compression by allowing each cluster to be
independently compressed with zlib.
Can I do that with guestfs_compress_out?