vxlan: do not create duplicated network for private gateway (apache#9232)

Author: weizhouapache

plugins/network-elements/vxlan/src/main/java/com/cloud/network/guru/VxlanGuestNetworkGuru.java

@@ -59,7 +59,7 @@ protected boolean canHandle(NetworkOffering offering, final NetworkType networkT
         // This guru handles only Guest Isolated network that supports Source nat service
         if (networkType == NetworkType.Advanced && isMyTrafficType(offering.getTrafficType()) &&
                 (offering.getGuestType() == Network.GuestType.Isolated || offering.getGuestType() == Network.GuestType.L2) &&
-                isMyIsolationMethod(physicalNetwork)) {
+                isMyIsolationMethod(physicalNetwork) && !offering.isSystemOnly()) {
             return true;
         } else {
             s_logger.trace("We only take care of Guest networks of type   " + GuestType.Isolated + " or " + GuestType.L2 + " in zone of type " + NetworkType.Advanced);