Enforce VM uniqueness when defining

Closes #56

client-common/vm.py

@@ -166,6 +166,10 @@ def define_vm(zk_conn, config_data, target_node, node_limit, node_selector, node
     dom_uuid = parsed_xml.uuid.text
     dom_name = parsed_xml.name.text
 
+    # Ensure that the UUID and name are unique
+    if searchClusterByUUID(dom_uuid) or searchClusterByName(dom_name):
+        return False, 'ERROR: Specified VM "{}" or UUID "{}" matches an existing VM on the cluster'.format(dom_name, dom_uuid)
+
     if not target_node:
         target_node = common.findTargetNode(zk_conn, dom_uuid)
     else: