storage: allow interleave in volume XML
    
    The RNG grammar did not allow arbitrary interleaving, which makes
    it harder than necessary to create a new volume from handwritten XML.
    (Compare also to commit caf516db for pools).
    
    * docs/schemas/storagevol.rng: Support interleaving.
    * tests/storagevolxml2xmlin/vol-file-backing.xml: Test it.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>