Virtio-net: Replace the hardcode 6 with defined ETN_ALEN

hw/virtio-net.h:
    #define ETH_ALEN    6
ETH_ALEN was defined by commit 7967406801

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-net.h

@@ -54,8 +54,8 @@
 
 struct virtio_net_config
 {
-    /* The config defining mac address (6 bytes) */
-    uint8_t mac[6];
+    /* The config defining mac address ($ETH_ALEN bytes) */
+    uint8_t mac[ETH_ALEN];
     /* See VIRTIO_NET_F_STATUS and VIRTIO_NET_S_* above */
     uint16_t status;
 } __attribute__((packed));