scripts/simplebench: compare write request performance

The script 'bench_write_req.py' allows comparing performances of write
request for two qemu-img binary files.
An example with (qemu-img binary 1) and without (qemu-img binary 2) the
applied patch "qcow2: skip writing zero buffers to empty COW areas"
(git commit ID: c8bb23cb) has the following results:

SSD:
----------------  -------------------  -------------------
                  <qemu-img binary 1>  <qemu-img binary 2>
<cluster front>   0.10 +- 0.00         8.16 +- 0.65
<cluster middle>  0.10 +- 0.00         7.37 +- 1.10
<cross cluster>   7.40 +- 1.08         21.97 +- 4.19
<cluster 64K>     2.14 +- 0.94         8.48 +- 1.66
----------------  -------------------  -------------------
HDD:
----------------  -------------------  -------------------
                  <qemu-img binary 1>  <qemu-img binary 2>
<cluster front>   2.30 +- 0.01         6.19 +- 0.06
<cluster middle>  2.20 +- 0.09         6.20 +- 0.06
<cross cluster>   8.32 +- 0.16         8.26 +- 0.14
<cluster 64K>     8.20 +- 0.05         6.26 +- 0.10
----------------  -------------------  -------------------

Suggested-by: Denis V. Lunev <den@openvz.org>
Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1594741846-475697-1-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>