Fix VLAN info not added to interfacesByIdx map

VLAN interfaces were initially created with a temporary index, which
only updated the `interfaces` map and not `interfacesByIdx`. This change
updates `interfacesByIdx` once `createInterface` is triggered after VLAN
creation.

Tested:
1. Enabled VLAN.
2. Verified VLAN IP by launching WebUI with VLAN IP.
3. Checked VLAN D-Bus objects are created.
4. Disabled VLAN.
5. Checked VLAN D-Bus objects are deleted.

Change-Id: I077adc400ff1248c9c7dda726206f9c54d8fb361
Signed-off-by: Rashid MP <rashidmp@ami.com>
diff --git a/src/network_manager.cpp b/src/network_manager.cpp
index 5991611..f5f16cc 100644
--- a/src/network_manager.cpp
+++ b/src/network_manager.cpp
@@ -189,6 +189,11 @@
         auto it = interfaces.find(*info.intf.name);
         if (it != interfaces.end())
         {
+            if (info.intf.vlan_id)
+            {
+                interfacesByIdx.insert_or_assign(info.intf.idx,
+                                                 it->second.get());
+            }
             it->second->updateInfo(info.intf);
             return;
         }