virtio-devices: Custom EpollHelper::run/VirtioCommon:reset for fuzz

It provides fuzzer a reliable way to wait for a sequence of events
to complete for virtio-devices while not using a fixed timeout to
maintain the full speed of fuzzing.

Take virtio-block as an example, the 'queue event' with a valid
available queue setup can trigger a 'completion event'. This is a
meaningful virtio-block code path of processing guest inputs which is
our target for fuzzing virtio devices.

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

virtio-devices/src/device.rs

@@ -280,6 +280,7 @@ impl VirtioCommon {
         Ok(())
     }
 
+    #[cfg(not(fuzzing))]
     pub fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
         // We first must resume the virtio thread if it was paused.
         if self.pause_evt.take().is_some() {
@@ -303,6 +304,20 @@ impl VirtioCommon {
         Some(self.interrupt_cb.take().unwrap())
     }
 
+    #[cfg(fuzzing)]
+    // Wait for the worker thread to finish and return
+    pub fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
+        if let Some(mut threads) = self.epoll_threads.take() {
+            for t in threads.drain(..) {
+                if let Err(e) = t.join() {
+                    error!("Error joining thread: {:?}", e);
+                }
+            }
+        }
+
+        None
+    }
+
     pub fn dup_eventfds(&self) -> (EventFd, EventFd) {
         (
             self.kill_evt.as_ref().unwrap().try_clone().unwrap(),