image: Create zstd-compressed qcow2 images.

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

gnu/build/image.scm

@@ -181,6 +181,7 @@ ROOT directory to populate the image."
              ;; The maximum number of co-routines is 16.
              "-m" (number->string (min 16 (parallel-job-count)))
              "-O" "qcow2"
+             "-o" "compression_type=zstd"
              image output))
     (else
      (copy-file image output))))