image: Honor parallel job count for the number of qemu-img co-routines used.

This doesn't seem to change anything currently (the default is 8).  Perhaps
useful in the future, for example if the compressor used by qemu-img starts
honoring it.

* gnu/build/image.scm (convert-disk-image): Specify the '-m' option.

Change-Id: Ia288c85af8a0621d61cf6b9f0d3f73f5a3026d66

gnu/build/image.scm

@@ -178,7 +178,10 @@ ROOT directory to populate the image."
   (case format
     ((compressed-qcow2)
      (invoke "qemu-img" "convert" "-c" "-f" "raw"
-             "-O" "qcow2" image output))
+             ;; The maximum number of co-routines is 16.
+             "-m" (number->string (min 16 (parallel-job-count)))
+             "-O" "qcow2"
+             image output))
     (else
      (copy-file image output))))