Implement kvm install latest

### kvm.sh
* Added latest argument to kvm install, which will install the latest kvm version (but not set the default)
* Added -p (persitant) switch to kvm install, and implemented kvm upgrade as kvm install latest -p
* Fixed some small issues when missing alias folder
* Fixed install to exit when package is not found in feed

### kvm.ps1
 *  Added latest argument to kvm install, which will install the latest kvm version (but not set the default)

fixes https://github.com/aspnet/KRuntime/issues/318