Fixes: add read-zeroes to 051.out

Fixes: add read-zeroes to 051.out
The patch "iotests: Set read-zeroes on in null block driver for Valgrind"
with the commit ID a6862418fec4072 needs the change in 051.out when
compared against on the s390 system.

Fixes: a6862418fec40727b392c86dc13d9ec980efcb15
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>