vvfat: use bdrv_new() to allocate BlockDriverState

we need bdrv_new() to properly initialize BDS, don't allocate memory
manually.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/vvfat.c

@@ -2943,7 +2943,7 @@ static int enable_write_target(BDRVVVFATState *s)
     unlink(s->qcow_filename);
 #endif
 
-    s->bs->backing_hd = calloc(sizeof(BlockDriverState), 1);
+    s->bs->backing_hd = bdrv_new("");
     s->bs->backing_hd->drv = &vvfat_write_target;
     s->bs->backing_hd->opaque = g_malloc(sizeof(void*));
     *(void**)s->bs->backing_hd->opaque = s;