isabus-bridge needs to be created by isa_bus_new(), and won't
work with -device, as it won't create the TYPE_ISA_BUS bus
itself. Remove the user_creatable flag from the device class.

Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-13-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>