block/null: Expose read-zeroes option in QAPI schema

Commit cd219eb1 added the read-zeroes option for the null-co and
null-aio block driver, but forgot to add them to the QAPI schema.
Therefore, this option wasn't available in -blockdev and blockdev-add
until now.

Add the missing option in the schema to make it available there, too.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>