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: NEric Blake <eblake@redhat.com>