qdev: Fix crash when using non-device class name on -global

This fixes the following crash:

    $ qemu-system-x86_64 -global container.xxx=y
    hw/core/qdev-properties-system.c:399:qdev_add_one_global: Object 0x7f7eff234100 is not an instance of type device
    Aborted (core dumped)

New behavior will be to just warn, just like when non-existing clas
names are used:

    $ qemu-system-x86_64 -global container.xxx=y
    qemu-system-x86_64: Warning: "-global container.xxx=y" not used
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Tested-by: NDon Slutz <dslutz@verizon.com>

hw/core/qdev-properties-system.c

@@ -394,7 +394,8 @@ static int qdev_add_one_global(QemuOpts *opts, void *opaque)
     g->driver   = qemu_opt_get(opts, "driver");
     g->property = qemu_opt_get(opts, "property");
     g->value    = qemu_opt_get(opts, "value");
-    oc = object_class_by_name(g->driver);
+    oc = object_class_dynamic_cast(object_class_by_name(g->driver),
+                                   TYPE_DEVICE);
     if (oc) {
         DeviceClass *dc = DEVICE_CLASS(oc);