Resolve symlinks in sdk paths and improve default simulator selection approach.

In this commit 2 problems are resolved, both could happen only if `-destination` option is not specified:
1) If xcodebuild is using latest sdk then sdk path could contain symlinks which should be resolved before trying to find appropriate `DTiPhoneSimulatorSystemRoot` instance to use. For example, in Xcode 6 last path component of `/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk` is symlink pointing to `iPhoneSimulator.sdk` in the same directory.
2) It could happen that `SimRuntime` supports `SimDeviceType` but there is no available configuration of device with that runtime and device type. It is better to loop through available devices and choose the one that is available.

This commit should resolver issues mentioned in https://github.com/facebook/xctool/issues/405.