virtio: make virtio_bus const

Now that the driver core can properly handle constant struct bus_type,
move the virtio_bus variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Message-Id: <20240204-bus_cleanup-virtio-v1-1-3bcb2212aaa0@marliere.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jason Wang <jasowang@redhat.com>