vm-virtio: Make all virtio devices potentially multi-threaded

Although only the block and net virtio devices can actually be multi
threaded (for now), handling them as special cases makes the code more
complex.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-virtio/src/vsock/device.rs

@@ -383,7 +383,7 @@ pub struct Vsock<B: VsockBackend> {
     acked_features: u64,
     queue_evts: Option<Vec<EventFd>>,
     interrupt_cb: Option<Arc<dyn VirtioInterrupt>>,
-    epoll_thread: Option<thread::JoinHandle<result::Result<(), DeviceError>>>,
+    epoll_thread: Option<Vec<thread::JoinHandle<result::Result<(), DeviceError>>>>,
     paused: Arc<AtomicBool>,
 }
 
@@ -554,15 +554,18 @@ where
         };
 
         let paused = self.paused.clone();
+        let mut epoll_threads = Vec::new();
         thread::Builder::new()
             .name("virtio_vsock".to_string())
             .spawn(move || handler.run(paused))
-            .map(|thread| self.epoll_thread = Some(thread))
+            .map(|thread| epoll_threads.push(thread))
             .map_err(|e| {
                 error!("failed to clone the vsock epoll thread: {}", e);
                 ActivateError::BadActivate
             })?;
 
+        self.epoll_thread = Some(epoll_threads);
+
         Ok(())
     }