qemu-iotests/199: change discard patterns

qemu-iotests/199: change discard patterns

iotest 199 works too long because of many discard operations. At the
same time, postcopy period is very short, in spite of all these
efforts.

So, let's use less discards (and with more interesting patterns) to
reduce test timing. In the next commit we'll increase postcopy period.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727194236.19551-6-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>