KVM: return null state instead of Disconnected when investigate a host without NFS (#10515)

Author: weizhouapache

plugins/hypervisors/kvm/src/main/java/com/cloud/ha/KVMInvestigator.java

@@ -92,7 +92,7 @@ public Status isAgentAlive(Host agent) {
         if (!storageSupportHA) {
             s_logger.warn(
                     "Agent investigation was requested on host " + agent + ", but host does not support investigation because it has no NFS storage. Skipping investigation.");
-            return Status.Disconnected;
+            return null;
         }
 
         Status hostStatus = null;