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: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: Don Slutz <dslutz@verizon.com>
