virtio-devices: Introduce WorkerThreads handle

Take the recent thread refactor further. Bundle a device's worker
JoinHandles together with the kill event that stops them into a single
WorkerThreads value, owned by VirtioCommon. Its Drop signals the workers
to exit, unparks any parked for migration, and joins them.

This makes a detached/leaked worker unrepresentable. reset(),
wait_for_epoll_threads() and VhostUserCommon::shutdown() now happen when
dropping the WorkerThreads, and the unpark-before-join teardown now
lives in one place.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Dylan Reid <dgreid@fb.com>
Signed-off-by: Rob Bradford <rbradford@meta.com>

virtio-devices/src/vsock/device.rs

@@ -426,19 +426,6 @@ where
     }
 }
 
-impl<B> Drop for Vsock<B>
-where
-    B: VsockBackend,
-{
-    fn drop(&mut self) {
-        if let Some(kill_evt) = self.common.kill_evt.take() {
-            // Ignore the result because there is nothing we can do about it.
-            let _ = kill_evt.write(1);
-        }
-        self.common.wait_for_epoll_threads();
-    }
-}
-
 impl<B> VirtioDevice for Vsock<B>
 where
     B: VsockBackend + Sync + 'static,