Add '-real xpath' as a supported iOS strategy.

Currently '-real xpath' is included as a valid strategy in [ios.js](https://github.com/appium/appium/blob/master/lib/devices/ios/ios.js#L92).  However, it's not exposed in [common.js](https://github.com/appium/appium/blob/master/lib/devices/common.js#L210).  This pull request is supposed to fix such an issue so that the client side won't run into the "Invalid locator strategy" error.