The RNG grammar did not allow arbitrary interleaving, which makes
it harder than necessary to create a new pool from handwritten XML.

* docs/schemas/storagepool.rng: Allow interleaving.
* tests/storagepoolxml2xmlin/pool-sheepdog.xml: Test interleave.
* tests/storagepoolxml2xmlin/pool-iscsi-auth.xml: Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>