virtio-devices: Print out worker thread errors

As we never join the spawned virtio-devices worker threads, the error
returned from each worker thread is lost. For now, we simply print out
the error from each worker thread.

Fixes: #1551

Signed-off-by: Bo Chen <chen.bo@intel.com>

virtio-devices/src/mem.rs

@@ -689,7 +689,7 @@ pub struct Mem {
     config: Arc<Mutex<VirtioMemConfig>>,
     queue_evts: Option<Vec<EventFd>>,
     interrupt_cb: Option<Arc<dyn VirtioInterrupt>>,
-    epoll_threads: Option<Vec<thread::JoinHandle<result::Result<(), EpollHelperError>>>>,
+    epoll_threads: Option<Vec<thread::JoinHandle<()>>>,
     paused: Arc<AtomicBool>,
     paused_sync: Arc<Barrier>,
 }
@@ -854,7 +854,11 @@ impl VirtioDevice for Mem {
         let mut epoll_threads = Vec::new();
         thread::Builder::new()
             .name("virtio_mem".to_string())
-            .spawn(move || handler.run(paused, paused_sync))
+            .spawn(move || {
+                if let Err(e) = handler.run(paused, paused_sync) {
+                    error!("Error running worker: {:?}", e);
+                }
+            })
             .map(|thread| epoll_threads.push(thread))
             .map_err(|e| {
                 error!("failed to clone virtio-mem epoll thread: {}", e);