vring: do not take &mut self for VringT

Backend::handle_event() takes an argument of `vrings: &[V]` with
`V: VringT`, so methods of VringT should not take `&mut self`.

Signed-off-by: Liu Jiang <[email protected]>

Author: jiangliu

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 79.1,
+  "coverage_score": 78.8,
   "exclude_path": "",
   "crate_features": ""
 }