kvm: Return capitalized NONE when no disk cache mode is set

This caused Instances not to start under rare circumstances

plugins/hypervisors/kvm/src/com/cloud/hypervisor/kvm/resource/LibvirtVMDef.java

@@ -431,7 +431,7 @@ public class LibvirtVMDef {
             @Override
             public String toString() {
                 if (_diskCacheMode == null) {
-                    return "none";
+                    return "NONE";
                 }
                 return _diskCacheMode;
             }