Image processing speedup and long commands support

Hi guys,

the first pull request makes it possible to execute even longer commands, than it was possible with the bash workaround.

the second one speedups the image processing in case we handle many disks (>20).

the third one uses those capabilities and removes the workarounds, which made it possible to run this test without this support.

Ad the _parallelize the image processing_ commit, there are details from the test run:
1 - 16 threads on 4 CPU host, create all disk, than raise exception (+ the postprocess)
(1/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (30.03 s)
(2/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (17.48 s)
(3/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (14.54 s)
(4/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (14.04 s)
(5/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (13.00 s)
(6/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.59 s)
(7/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (11.64 s)
(8/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (11.59 s)
(9/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.63 s)
(10/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.63 s)
(11/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.65 s)
(12/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (11.65 s)
(13/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.72 s)
(14/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.27 s)
(15/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.80 s)
(16/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (12.38 s)

1 - 16 threads on 4 CPU host, 100th disk raises exception (+ the postprocess)
(1/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (15.10 s)
(2/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (9.00 s)
(3/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.99 s)
(4/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.53 s)
(5/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.54 s)
(6/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (6.55 s)
(7/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.10 s)
(8/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (6.57 s)
(9/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.11 s)
(10/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.08 s)
(11/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.22 s)
(12/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.14 s)
(13/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (7.23 s)
(14/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (6.28 s)
(15/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (6.82 s)
(16/16) multi_disk.virtio_scsi_variants.multi_lun: FAIL (4.41 s)