vhost_user_backend: Don't report out socket broken errors

This is a perfectly acceptable situation as it causes the backend to
exit because the VMM has closed the connection. This addresses the
rather ugly reporting of errors from the backend that appears
interleaved with the output from the VMM.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vhost_user_backend/src/lib.rs

@@ -174,7 +174,11 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
     /// terminate.
     pub fn wait(&mut self) -> Result<()> {
         if let Some(handle) = self.main_thread.take() {
-            handle.join().map_err(Error::WaitDaemon)?
+            match handle.join().map_err(Error::WaitDaemon)? {
+                Ok(()) => Ok(()),
+                Err(Error::HandleRequest(VhostUserError::SocketBroken(_))) => Ok(()),
+                Err(e) => Err(e),
+            }
         } else {
             Ok(())
         }