virttest.aexpect: Support commands longer than 2MB

Current kernels support unlimited argument length, the only limit is
stack size. The default stack size is usually 8KB which is sufficient
for ~2MB commands. Use ulimit to increase these limits. The multi_disk
in some configurations attacks the 2MB barier and in the future we
might see this comming (unless we switch to -readconfig).

NOTE: Only superuser can increase the hard limit, anyway if the command
      fails the test continues and tries to execute the cmd anyway.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.env_process: Parallelize the image processing

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: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk: Tweak the config file of virtio_scsi_variants

Use the unified configs (scsiid=>unit, lun=>port) and store the images
in the usual location. The location '/tmp/%s' was used before because
python was unable to execute that long command and this prefix shorten
it significantly. Now autotest supports the way to deal with long
commands so we can finally store the images in the right location.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
More PEP8 fixes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>