Xcode 5.1.1 cannot run on device with iOS 8.0 version
You need to gather symbols from device before connecting it to Xcode 5. Download and install Xcode 6 side by side with older Xcode. Connect iOS8 device to Xcode 6. Wait for it to collect symbols and see if device is ready for development. From now on Xcode 5 is able to "see" the device properly.