schemas: rng: Make secret optional in storage vol
schemas: rng: Make secret optional in storage vol

For a qcow2 storage volume with luks encryption created by qemu-img, its
dumped storage vol XML has no secret element in encryption:
<volume type='file'>
    ...
    <encryption format='luks'>
    </encryption>
    ...
</volume>

That will cause a failure in rng validation. Fix that validation failure.

Signed-off-by: Han Han <2f3eec96101b6aa65baff01c5b79fa5c74aa8195@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt