iotests: Fix throttling in 030

iotests: Fix throttling in 030

Currently, TestParallelOps in 030 creates images that are too small for
job throttling to be effective.  This is reflected by the fact that it
never undoes the throttling.

Increase the image size and undo the throttling when the job should be
completed.  Also, add throttling in test_overlapping_4, or the jobs may
not be so overlapping after all.  In fact, the error usually emitted
here is that node2 simply does not exist, not that overlapping jobs are
not allowed -- the fact that this job ignores the exact error messages
and just checks the error class is something that should be fixed in a
follow-up patch.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Tested-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190703172813.6868-8-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>