We want iotests pass with both the default zlib compression and with
IMGOPTS='compression_type=zstd'.

Actually the only test that is interested in real compression type in
test output is 287 (test for qcow2 compression type), so implement
specific option for it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211223160144.1097696-17-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>