qemu-img: In "map", use the returned "file" from bdrv_get_block_status

Now all drivers should return a correct "file", we can make use of it,
even with the recursion into backing chain above.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-id: 1453780743-16806-15-git-send-email-famz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

qemu-img.c

@@ -2236,7 +2236,7 @@ static int get_block_status(BlockDriverState *bs, int64_t sector_num,
     e->flags = ret & ~BDRV_BLOCK_OFFSET_MASK;
     e->offset = ret & BDRV_BLOCK_OFFSET_MASK;
     e->depth = depth;
-    e->bs = bs;
+    e->bs = file;
     return 0;
 }