Add iblock support and tests

* Somehow iblock's policy file did not make it to the repo. Fixed.
* Added config_Sample_6 test for iblock: uses the scsi_dbug module to create
  block devices, exposed to config_sample_* tests as /tmp/test_blockdev_[0-3]
Updated backstore policy files defaults and types

* Defaults now match kernel 3.13
* Fixed a couple of type mismatches
* Updated test config sample to reflect these