tests: Set read-zeroes on for null-co driver

tests: Set read-zeroes on for null-co driver

This patch is to reduce the number of Valgrind report messages about
using uninitialized memory with the null-co driver. It helps to filter
real memory issues and is the same work done for the iotests with the
commit ID a6862418fec4072.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Message-Id: <1564404360-733987-1-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>