iotests: Test backup job with two guest writes

Perform two guest writes to not yet backed up areas of an image, where
the former touches an inner area of the latter.

Before HEAD^, copy offloading broke this in two ways:
(1) The target image differs from the reference image (what the source
    was when the backup started).
(2) But you will not see that in the failing output, because the job
    offset is reported as being greater than the job length.  This is
    because one cluster is copied twice, and thus accounted for twice,
    but of course the job length does not increase.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190801173900.23851-3-mreitz@redhat.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Tested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

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