virtio-devices: block: make shutdown join the worker

eject_device calls shutdown, but Block doesn't implement it, so the
worker thread was never joined. Drop for Block does not help either,
VirtioPciDevice keeps an Arc<Mutex<Block>> past eject, so Drop never
runs. The async worker keeps completing in-flight I/O into the guest RAM
that backed it when issued.

Implement shutdown() to call wait_for_epoll_threads(), which drops the
WorkerThreads handle and ensures that worker's io completes.

Signed-off-by: Dylan Reid <dgreid@fb.com>

virtio-devices/src/block.rs

@@ -1167,6 +1167,10 @@ impl VirtioDevice for Block {
         event!("virtio-device", "reset", "id", &self.id);
     }
 
+    fn shutdown(&mut self) {
+        self.common.wait_for_epoll_threads();
+    }
+
     fn counters(&self) -> Option<HashMap<&'static str, Wrapping<u64>>> {
         let mut counters = HashMap::new();