hw/pci: Fix crash when running QEMU with "-nic model=rocker"

QEMU currently aborts when being started with "-nic model=rocker" or with
"-net nic,model=rocker". This happens because the "rocker" device is not
a normal NIC but a switch, which has different properties. Thus we should
only consider real NIC devices for "-nic" and "-net". These devices can
be identified by the "netdev" property, so check for this property before
adding the device to the list.
Reported-by: NMichael Tokarev <mjt@tls.msk.ru>
Fixes: 52310c3f ("net: allow using any PCI NICs in -net or -nic")
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20200527153152.9211-1-thuth@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pci/pci.c

@@ -1891,7 +1891,18 @@ PCIDevice *pci_nic_init_nofail(NICInfo *nd, PCIBus *rootbus,
         if (test_bit(DEVICE_CATEGORY_NETWORK, dc->categories) &&
             dc->user_creatable) {
             const char *name = object_class_get_name(list->data);
-            g_ptr_array_add(pci_nic_models, (gpointer)name);
+            /*
+             * A network device might also be something else than a NIC, see
+             * e.g. the "rocker" device. Thus we have to look for the "netdev"
+             * property, too. Unfortunately, some devices like virtio-net only
+             * create this property during instance_init, so we have to create
+             * a temporary instance here to be able to check it.
+             */
+            Object *obj = object_new_with_class(OBJECT_CLASS(dc));
+            if (object_property_find(obj, "netdev", NULL)) {
+                g_ptr_array_add(pci_nic_models, (gpointer)name);
+            }
+            object_unref(obj);
         }
         next = list->next;
         g_slist_free_1(list);