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/hw/mipsnet.c b/hw/mipsnet.c
index 15761b1..ff6bf7f 100644
--- a/hw/mipsnet.c
+++ b/hw/mipsnet.c
@@ -64,7 +64,7 @@
 
 static int mipsnet_can_receive(NetClientState *nc)
 {
-    MIPSnetState *s = DO_UPCAST(NICState, nc, nc)->opaque;
+    MIPSnetState *s = qemu_get_nic_opaque(nc);
 
     if (s->busy)
         return 0;
@@ -73,7 +73,7 @@
 
 static ssize_t mipsnet_receive(NetClientState *nc, const uint8_t *buf, size_t size)
 {
-    MIPSnetState *s = DO_UPCAST(NICState, nc, nc)->opaque;
+    MIPSnetState *s = qemu_get_nic_opaque(nc);
 
     trace_mipsnet_receive(size);
     if (!mipsnet_can_receive(nc))
@@ -211,7 +211,7 @@
 
 static void mipsnet_cleanup(NetClientState *nc)
 {
-    MIPSnetState *s = DO_UPCAST(NICState, nc, nc)->opaque;
+    MIPSnetState *s = qemu_get_nic_opaque(nc);
 
     s->nic = NULL;
 }