server: acquire IPv4 address when add secondary IP to nic if IP is not specified (#3635)

After commit fbf4884, admin need to specify an ipv4 or ipv6 addresses when add IP to nic which breaks backward compatibity. If IP is not specified, a IPv4 address should be returned.

Author: ustcweizhou

server/src/main/java/com/cloud/network/NetworkServiceImpl.java

@@ -732,11 +732,10 @@ public NicSecondaryIp allocateSecondaryGuestIP(final long nicId, IpAddresses req
             }
 
             try {
-                if (ipv4Address != null) {
-                    ipaddr = _ipAddrMgr.allocatePublicIpForGuestNic(network, podId, ipOwner, ipv4Address);
-                }
                 if (ipv6Address != null) {
                     ip6addr = ipv6AddrMgr.allocatePublicIp6ForGuestNic(network, podId, ipOwner, ipv6Address);
+                } else {
+                    ipaddr = _ipAddrMgr.allocatePublicIpForGuestNic(network, podId, ipOwner, ipv4Address);
                 }
                 if (ipaddr == null && ipv6Address == null) {
                     throw new InvalidParameterValueException("Allocating ip to guest nic " + nicId + " failed");