iotests: 257: prepare for backup over block-copy

iotests: 257: prepare for backup over block-copy

Iotest 257 dumps a lot of in-progress information of backup job, such
as offset and bitmap dirtiness. Further commit will move backup to be
one block-copy call, which will introduce async parallel requests
instead of plain cluster-by-cluster copying. To keep things
deterministic, allow only one worker (only one copy request at a time)
for this test.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210116214705.822267-15-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>