xen: Fix PV-on-HVM

In the context of PV-on-HVM under Xen, the emulated nics are supposed to be
unplug before the guest drivers are initialized, when the guest write to a
specific IO port.

Without this patch, the guest end up with two nics with the same MAC, the
emulated nic and the PV nic.
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/xen_platform.c

@@ -87,7 +87,10 @@ static void unplug_nic(PCIBus *b, PCIDevice *d)
 {
     if (pci_get_word(d->config + PCI_CLASS_DEVICE) ==
             PCI_CLASS_NETWORK_ETHERNET) {
-        qdev_unplug(&(d->qdev), NULL);
+        /* Until qdev_free includes a call to object_unparent, we call it here
+         */
+        object_unparent(&d->qdev.parent_obj);
+        qdev_free(&d->qdev);
     }
 }