Some cases create many disks, which takes a long time, where most
time consumes the handling around cmd execution. This patch parallelize
the image processing in case we deal with over 20 disks. It spawns
$no_cpus * 2 threads (or less in case it would be inefficient).

The performance improvement was significant (30=>12s on 4CPU system).
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>