block: use correct filename

The content filename point to may be erased by qemu_opts_absorb_qdict()
in raw_open_common() in drv->bdrv_file_open()

So it's better to use bs->filename.

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -824,8 +824,8 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
 
 #ifndef _WIN32
     if (bs->is_temporary) {
-        assert(filename != NULL);
-        unlink(filename);
+        assert(bs->filename[0] != '\0');
+        unlink(bs->filename);
     }
 #endif
     return 0;