Fix enable-accessibility in CoreSimulator

After installing a new Xcode 6, it is necessary to restart your machine, even if you are doing a side-by-side install.

Xcode 6.1 adds iOS 8.1 Simulators.  On machines with side-by-side Xcode installs, different versions of instruments will report different 'known' simulators.

SimControl will now skip invalid `CoreSimulator/Devices/<UDID>` + `instruments -s devices` pairs.

Thanks @fabb for digging into this.