virtio-net: avoid call tap_enable when there's only one queue

We call tap_enable() even if for multiqueue is not enabled. This is
wrong since it should be used for multiqueue codes to enable a
disabled queue. Fixing this by only calling this when multiqueue is
used.

Fixes: 16dbaf90 ("tap: support enabling or disabling a queue")
Reported-by: NAndrew Baumann <Andrew.Baumann@microsoft.com>
Tested-by: NAndrew Baumann <Andrew.Baumann@microsoft.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NJason Wang <jasowang@redhat.com>

hw/net/virtio-net.c

@@ -510,6 +510,10 @@ static int peer_attach(VirtIONet *n, int index)
         return 0;
     }
 
+    if (n->max_queues == 1) {
+        return 0;
+    }
+
     return tap_enable(nc->peer);
 }