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
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