net: introduce qemu_get_queue()

To support multiqueue, the patch introduce a helper qemu_get_queue()
which is used to get the NetClientState of a device. 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/xilinx_axienet.c b/hw/xilinx_axienet.c
index 51c2896..a7e8e2c 100644
--- a/hw/xilinx_axienet.c
+++ b/hw/xilinx_axienet.c
@@ -826,7 +826,7 @@
         buf[write_off + 1] = csum & 0xff;
     }
 
-    qemu_send_packet(&s->nic->nc, buf, size);
+    qemu_send_packet(qemu_get_queue(s->nic), buf, size);
 
     s->stats.tx_bytes += size;
     s->regs[R_IS] |= IS_TX_COMPLETE;
@@ -853,7 +853,7 @@
     qemu_macaddr_default_if_unset(&s->conf.macaddr);
     s->nic = qemu_new_nic(&net_xilinx_enet_info, &s->conf,
                           object_get_typename(OBJECT(dev)), dev->qdev.id, s);
-    qemu_format_nic_info_str(&s->nic->nc, s->conf.macaddr.a);
+    qemu_format_nic_info_str(qemu_get_queue(s->nic), s->conf.macaddr.a);
 
     tdk_init(&s->TEMAC.phy);
     mdio_attach(&s->TEMAC.mdio_bus, &s->TEMAC.phy, s->c_phyaddr);