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.

vmm/src/lib.rs

@@ -43,6 +43,7 @@ use std::os::unix::net::UnixListener;
 use std::os::unix::net::UnixStream;
 use std::panic::AssertUnwindSafe;
 use std::path::PathBuf;
+use std::rc::Rc;
 use std::sync::mpsc::{Receiver, RecvError, SendError, Sender};
 use std::sync::{Arc, Mutex};
 use std::time::Instant;
@@ -347,9 +348,9 @@ pub fn start_vmm_thread(
                 vmm.setup_signal_handler()?;
 
                 vmm.control_loop(
-                    Arc::new(api_receiver),
+                    Rc::new(api_receiver),
                     #[cfg(feature = "guest_debug")]
-                    Arc::new(gdb_receiver),
+                    Rc::new(gdb_receiver),
                 )
             })
             .map_err(Error::VmmThreadSpawn)?
@@ -1791,8 +1792,8 @@ impl Vmm {
 
     fn control_loop(
         &mut self,
-        api_receiver: Arc<Receiver<ApiRequest>>,
-        #[cfg(feature = "guest_debug")] gdb_receiver: Arc<Receiver<gdb::GdbRequest>>,
+        api_receiver: Rc<Receiver<ApiRequest>>,
+        #[cfg(feature = "guest_debug")] gdb_receiver: Rc<Receiver<gdb::GdbRequest>>,
     ) -> Result<()> {
         const EPOLL_EVENTS_LEN: usize = 100;