build: Fix build breakage detected by buildbot

More details:
http://buildbot.b1-systems.de/qemu/builders/xen_unstable/builds/83/steps/compile/logs/stdio

VLANState has been removed since commit a005d073, so "vlan id" should be
fetched using net_hub_id_for_client().
Signed-off-by: NDunrong Huang <riegamaths@gmail.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/xen_backend.h

@@ -4,6 +4,7 @@
 #include "xen_common.h"
 #include "sysemu.h"
 #include "net.h"
+#include "net/hub.h"
 
 /* ------------------------------------------------------------- */
 

hw/xen_devconfig.c

@@ -123,19 +123,21 @@ int xen_config_dev_nic(NICInfo *nic)
 {
     char fe[256], be[256];
     char mac[20];
+    int vlan_id = -1;
 
+    net_hub_id_for_client(nic->netdev, &vlan_id);
     snprintf(mac, sizeof(mac), "%02x:%02x:%02x:%02x:%02x:%02x",
              nic->macaddr.a[0], nic->macaddr.a[1], nic->macaddr.a[2],
              nic->macaddr.a[3], nic->macaddr.a[4], nic->macaddr.a[5]);
-    xen_be_printf(NULL, 1, "config nic %d: mac=\"%s\"\n", nic->vlan->id, mac);
-    xen_config_dev_dirs("vif", "qnic", nic->vlan->id, fe, be, sizeof(fe));
+    xen_be_printf(NULL, 1, "config nic %d: mac=\"%s\"\n", vlan_id, mac);
+    xen_config_dev_dirs("vif", "qnic", vlan_id, fe, be, sizeof(fe));
 
     /* frontend */
-    xenstore_write_int(fe, "handle",     nic->vlan->id);
+    xenstore_write_int(fe, "handle",     vlan_id);
     xenstore_write_str(fe, "mac",        mac);
 
     /* backend */
-    xenstore_write_int(be, "handle",     nic->vlan->id);
+    xenstore_write_int(be, "handle",     vlan_id);
     xenstore_write_str(be, "mac",        mac);
 
     /* common stuff */