server: update dhcp configurations in vrs while update default nic of running vms (#3205)

In virtual routers, there are different dnsmasq settings for default nic and non-default nic on vm.
We need to update dhcp informations on network vrs when default nic is changed.

For example, if 172.16.1.135 is non-default nic of vm VPC1-001-001, then

root@r-22-VM:~# cat /etc/dhcphosts.txt
02:00:1d:15:00:05,set:172_16_1_135,172.16.1.135,VPC1-001-001,710h
root@r-22-VM:~# cat /etc/dhcpopts.txt
172_16_1_135,3
172_16_1_135,6
172_16_1_135,15

If it is default nic,then

root@r-22-VM:~# cat /etc/dhcpopts.txt
root@r-22-VM:~# cat /etc/dhcphosts.txt
02:00:1d:15:00:05,172.16.1.135,VPC1-001-001,757h

Fixes #3201

Author: ustcweizhou

server/src/com/cloud/vm/UserVmManagerImpl.java

@@ -1448,6 +1448,19 @@ public UserVm updateDefaultNicForVirtualMachine(UpdateDefaultNicForVMCmd cmd) th
                     newNetworkOfferingId, null, 0L, VirtualMachine.class.getName(), vmInstance.getUuid(), vmInstance.isDisplay());
             UsageEventUtils.publishUsageEvent(EventTypes.EVENT_NETWORK_OFFERING_ASSIGN, vmInstance.getAccountId(), vmInstance.getDataCenterId(), vmInstance.getId(),
                     oldNicIdString, oldNetworkOfferingId, null, 0L, VirtualMachine.class.getName(), vmInstance.getUuid(), vmInstance.isDisplay());
+
+            if (vmInstance.getState() != State.Stopped) {
+                try {
+                    VirtualMachineProfile vmProfile = new VirtualMachineProfileImpl(vmInstance);
+                    User callerUser = _accountMgr.getActiveUser(CallContext.current().getCallingUserId());
+                    ReservationContext context = new ReservationContextImpl(null, null, callerUser, caller);
+                    DeployDestination dest = new DeployDestination(dc, null, null, null);
+                    _networkMgr.prepare(vmProfile, dest, context);
+                } catch (final Exception e) {
+                    s_logger.info("Got exception: ", e);
+                }
+            }
+
             return _vmDao.findById(vmInstance.getId());
         }