virtio-rng: Avoid default_backend refcount leak

QOM child properties take a reference to the object and release it when
the property is deleted.  Therefore we should unref the default_backend
after we have added it as a child property.

Cc: KONRAD Frederic <fred.konrad@greensocs.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
