build: Fix beta clippy issue (arc_with_non_send_sync)

warning: usage of `Arc<T>` where `T` is not `Send` or `Sync`
   --> virtio-devices/src/vsock/device.rs:376:22
    |
376 |             backend: Arc::new(RwLock::new(backend)),
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider using `Rc<T>` instead or wrapping `T` in a std::sync type like `Mutex<T>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#arc_with_non_send_sync
    = note: `#[warn(clippy::arc_with_non_send_sync)]` on by default

The vsock backend may be shared between threads, so the type `B` in
`Vsock` should be `VsockBackend` and `Sync`.

Considering that `api_receiver` and `gdb_receiver` are only used in vmm
threads, the `Arc` can be replaced by `Rc`.

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

virtio-devices/src/vsock/device.rs

@@ -333,7 +333,7 @@ impl VersionMapped for VsockState {}
 
 impl<B> Vsock<B>
 where
-    B: VsockBackend,
+    B: VsockBackend + Sync,
 {
     /// Create a new virtio-vsock device with the given VM CID and vsock
     /// backend.