virtio-devices: properly join all threads on Drop

This change is important to do a proper resource cleanup. We decided
to do this repetitive approach as VirtioCommon can't implement Drop
without major changes to the corresponding code. Also, devices such as
Net can't easily use the epoll_threads-abstraction from VirtioCommon as
it has multiple threads with different semantics.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

virtio-devices/src/vsock/device.rs

@@ -397,6 +397,7 @@ where
             // Ignore the result because there is nothing we can do about it.
             let _ = kill_evt.write(1);
         }
+        self.common.wait_for_epoll_threads();
     }
 }