net: introduce qemu_get_nic()

To support multiqueue, this patch introduces a helper qemu_get_nic() to get
NICState from a NetClientState. The following patches would refactor this helper
to support multiqueue.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/net/net.c b/net/net.c
index b986161..606e860 100644
--- a/net/net.c
+++ b/net/net.c
@@ -227,7 +227,7 @@
 
     nc = qemu_new_net_client(info, conf->peer, model, name);
 
-    nic = DO_UPCAST(NICState, nc, nc);
+    nic = qemu_get_nic(nc);
     nic->conf = conf;
     nic->opaque = opaque;
 
@@ -239,6 +239,18 @@
     return &nic->nc;
 }
 
+NICState *qemu_get_nic(NetClientState *nc)
+{
+    return DO_UPCAST(NICState, nc, nc);
+}
+
+void *qemu_get_nic_opaque(NetClientState *nc)
+{
+    NICState *nic = qemu_get_nic(nc);
+
+    return nic->opaque;
+}
+
 static void qemu_cleanup_net_client(NetClientState *nc)
 {
     QTAILQ_REMOVE(&net_clients, nc, next);
@@ -265,7 +277,7 @@
 {
     /* If there is a peer NIC, delete and cleanup client, but do not free. */
     if (nc->peer && nc->peer->info->type == NET_CLIENT_OPTIONS_KIND_NIC) {
-        NICState *nic = DO_UPCAST(NICState, nc, nc->peer);
+        NICState *nic = qemu_get_nic(nc->peer);
         if (nic->peer_deleted) {
             return;
         }
@@ -281,7 +293,7 @@
 
     /* If this is a peer NIC and peer has already been deleted, free it now. */
     if (nc->peer && nc->info->type == NET_CLIENT_OPTIONS_KIND_NIC) {
-        NICState *nic = DO_UPCAST(NICState, nc, nc);
+        NICState *nic = qemu_get_nic(nc);
         if (nic->peer_deleted) {
             qemu_free_net_client(nc->peer);
         }
@@ -297,7 +309,7 @@
 
     QTAILQ_FOREACH(nc, &net_clients, next) {
         if (nc->info->type == NET_CLIENT_OPTIONS_KIND_NIC) {
-            func(DO_UPCAST(NICState, nc, nc), opaque);
+            func(qemu_get_nic(nc), opaque);
         }
     }
 }