iotests/129: Limit mirror job's buffer size

iotests/129: Limit mirror job's buffer size

Issuing 'stop' on the VM drains all nodes.  If the mirror job has many
large requests in flight, this may lead to significant I/O that looks a
bit like 'stop' would make the job try to complete (which is what 129
should verify not to happen).

We can limit the I/O in flight by limiting the buffer size, so mirror
will make very little progress during the 'stop' drain.

(We do not need to do anything about commit, which has a buffer size of
512 kB by default; or backup, which goes cluster by cluster.  Once we
have asynchronous requests for backup, that will change, but then we can
fine-tune the backup job to only perform a single request on a very
small chunk, too.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210118105720.14824-9-mreitz@redhat.com>