Fix KVM references 

### For kvm.ps1
* Updated downloading message to display actual feed URL.
* Fixes #392

## For kvm.sh

* Renamed Nuget API Feed variable to KRE_FEED in line with terminology from windows kvm.ps1
* Only set KRE_FEED when it is not already set, allowing the user to override if necessary (in line with kvm.ps1)
* Fixed return value of _kvm_find_latest and terminated kvm when _kvm_find_latest does not find a valid version in the feed
Checks if the XML response from the feed contains a <a:Version> node, otherwise returns 1. The calling method will then print a message that it can't get the latest version and terminate. Previously it would try to install a Bogus version.