iotests: Set read-zeroes on in null block driver for Valgrind

iotests: Set read-zeroes on in null block driver for Valgrind

The Valgrind tool reports about the uninitialised buffer 'buf'
instantiated on the stack of the function guess_disk_lchs().
Pass 'read-zeroes=on' to the null block driver to make it deterministic.
The output of the tests 051, 186 and 227 now includes the parameter
'read-zeroes'. So, the benchmark output files are being changed too.

Suggested-by: Kevin Wolf <[email protected]>
Signed-off-by: Andrey Shinkevich <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>