If read-zeroes is not set, we did not report BDRV_BLOCK_DATA or
BDRV_BLOCK_ZERO. This is not consistent with other drivers and can
confuse users or other programs:

    % qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
    [{ "start": 0, "length": 1073741824, "depth": 0, "present": false, "zero": false, "data": false, "compressed": false}]

    % qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &

    % nbdinfo --map nbd://127.0.0.1
             0  1073741824    1  hole

With this change we report DATA in this case:

    % ./qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
    [{ "start": 0, "length": 1073741824, "depth": 0, "present": true, "zero": false, "data": true, "compressed": false, "offset": 0}]

    % ./qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &

    % nbdinfo --map nbd://127.0.0.1
             0  1073741824    0  data

Signed-off-by: Nir Soffer <nirsof@gmail.com>
---
 block/null.c         | 4 +---
 qapi/block-core.json | 5 +++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/block/null.c b/block/null.c
index dc0b1fdbd9..7ba87bd9a9 100644
--- a/block/null.c
+++ b/block/null.c
@@ -239,9 +239,7 @@ static int coroutine_fn null_co_block_status(BlockDriverState *bs,
     *map = offset;
     *file = bs;
 
-    if (s->read_zeroes) {
-        ret |= BDRV_BLOCK_ZERO;
-    }
+    ret |= s->read_zeroes ? BDRV_BLOCK_ZERO : BDRV_BLOCK_DATA;
     return ret;
 }
 
diff --git a/qapi/block-core.json b/qapi/block-core.json
index b1937780e1..7c95c9e36a 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3293,8 +3293,9 @@
 #     requests.  Default to zero which completes requests immediately.
 #     (Since 2.4)
 #
-# @read-zeroes: if true, reads from the device produce zeroes; if
-#     false, the buffer is left unchanged.
+# @read-zeroes: if true, emulate a sparse image, and reads from the
+#     device produce zeroes; if false, emulate an allocated image but
+#     reads from the device leave the buffer unchanged.
 #     (default: false; since: 4.1)
 #
 # Since: 2.9
-- 
2.39.5 (Apple Git-154)