qcow2: Report BDRV_BLOCK_ZERO more accurately in bdrv_co_block_status()

If a BlockDriverState supports backing files but has none then any
unallocated area reads back as zeroes.

bdrv_co_block_status() is only reporting this is if want_zero is true,
but this is an inexpensive test and there is no reason not to do it in
all cases.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <66fa0914a0e2b727ab6d1b63ca773d7cd29a9a9e.1603731354.git.berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -2282,17 +2282,17 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
 
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else if (want_zero && bs->drv->supports_backing) {
+    } else if (bs->drv->supports_backing) {
         BlockDriverState *cow_bs = bdrv_cow_bs(bs);
 
-        if (cow_bs) {
+        if (!cow_bs) {
+            ret |= BDRV_BLOCK_ZERO;
+        } else if (want_zero) {
             int64_t size2 = bdrv_getlength(cow_bs);
 
             if (size2 >= 0 && offset >= size2) {
                 ret |= BDRV_BLOCK_ZERO;
             }
-        } else {
-            ret |= BDRV_BLOCK_ZERO;
         }
     }