Below is what worked for me to get a few extra test devices (my carry phone runs the 10 beta) working with Xcode 7. Can't upgrade to 8 just yet. Working with a contractor and don't want to move too many pieces.
Xcode 7.3.1 and Beta 6 iOS 10 beta 6 (Beta 7 is out, after getting this to work, I don't think I'm upgrading just yet)
sudo ln -s /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0\ \(14A5339a\)/ /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
- iPhone: Settings > General > Reset > Network Settings
- Reboot iPhone (after it wakes back up from the network reset)
- Reboot Mac (restarting Xcode alone didn't work)
It these steps don't work for you, check out the gist for other options.