virtio blk: fix warning.

(cherry picked from commit 5c5dafdc)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-blk.c

@@ -247,6 +247,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
 
     bdrv_get_geometry(s->bs, &capacity);
     bdrv_get_geometry_hint(s->bs, &cylinders, &heads, &secs);
+    memset(&blkcfg, 0, sizeof(blkcfg));
     stq_raw(&blkcfg.capacity, capacity);
     stl_raw(&blkcfg.seg_max, 128 - 2);
     stw_raw(&blkcfg.cylinders, cylinders);