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?