network: Correct error reporting in listInterfaces()

There are two problems with error reporting here.
  1) If the upcast from resultingInterfaces to *grpc.Interfaces fails, we
     return (nil, err), but previous code ensures that err is nil at that
     point, so we return no error.

  2) Due to shadowing the err variable, if an error is reported by
     convertToInterfaces, we'll also return (nil, nil) in that case

fixes #3032

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

virtcontainers/kata_agent.go

@@ -672,13 +672,10 @@ func (k *kataAgent) listInterfaces() ([]*vcTypes.Interface, error) {
 		return nil, err
 	}
 	resultInterfaces, ok := resultingInterfaces.(*grpc.Interfaces)
-	if ok {
-		ifaces, err := k.convertToInterfaces(resultInterfaces.Interfaces)
-		if err == nil {
-			return ifaces, nil
-		}
+	if !ok {
+		return nil, fmt.Errorf("Unexpected type %T for interfaces", resultingInterfaces)
 	}
-	return nil, err
+	return k.convertToInterfaces(resultInterfaces.Interfaces)
 }
 
 func (k *kataAgent) listRoutes() ([]*vcTypes.Route, error) {