This should be about twice as fast as the default zlib compressor used for
qcow2, and potentially multi-thread aware given some improvements to qemu-img
in the future (zstd supports its but it's not currently used by QEMU).
* gnu/build/image.scm (convert-disk-image): Specify 'compression_type=zstd'
option.
Change-Id: Ie9c66f0c13e789ec863c95a5e549f035b6a17bf9