virtio-devices: modify or provide safety comments

Signed-off-by: Wei Liu <liuwe@microsoft.com>

virtio-devices/src/vhost_user/mod.rs

@@ -375,6 +375,7 @@ impl VhostUserCommon {
 
     pub fn shutdown(&mut self) {
         if let Some(vu) = &self.vu {
+            // SAFETY: trivially safe
             let _ = unsafe { libc::close(vu.lock().unwrap().socket_handle().as_raw_fd()) };
         }