vbox: fix vbox race with unregistervm

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

drivers/virtualbox/virtualbox.go

@@ -462,6 +462,8 @@ func (d *Driver) Remove() error {
 			return err
 		}
 	}
+	// vbox will not release it's lock immediately after the stop
+	time.Sleep(1 * time.Second)
 	return vbm("unregistervm", "--delete", d.MachineName)
 }