virtio_rng: replace custom backend API with UserCreatable.complete() callback

in addition fix default backend leak by releasing it if its
initialization failed.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
