Allows the initiator to use a variety of IQNs rather than just the
system IQN when creating iSCSI pools.
* docs/schemas/storagepool.rng: extends the syntax with <iqn name="..."/>
* src/conf/storage_conf.[ch]: read and stores the iqn name
* src/storage/storage_backend_iscsi.[ch]: implement the IQN selection
  when detected