block: use bdrv_has_zero_init to return BDRV_BLOCK_ZERO

Alternatively, this could use a "discard zeroes data" flag returned
by bdrv_get_info.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

diff --git a/block.c b/block.c
index 97406ec..aa9ec83 100644
--- a/block.c
+++ b/block.c
@@ -3075,6 +3075,7 @@
 {
     int64_t length;
     int64_t n;
+    int64_t ret;
 
     length = bdrv_getlength(bs);
     if (length < 0) {
@@ -3096,7 +3097,15 @@
         return BDRV_BLOCK_DATA;
     }
 
-    return bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum);
+    ret = bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (!(ret & BDRV_BLOCK_DATA) && bdrv_has_zero_init(bs)) {
+        ret |= BDRV_BLOCK_ZERO;
+    }
+    return ret;
 }
 
 /* Coroutine wrapper for bdrv_get_block_status() */