This adds a reasonably comprehensive test script for working with
disk based storage pools. It checks

 - Prevent re-formatting an existing partition table (currently
   missing this check in libvirt)
 - Formatting of a partiton table
 - Wiping a partition table (currently missing in libvirt)
 - Creating multiple storage volumes
 - Deleting storage volumes

* conf/default.cfg: add config entry for specifying host block
  devices that a test script can use
* lib/Sys/Virt/TCK.pm: Add API to get a host block device
* lib/Sys/Virt/TCK/StoragePoolBuilder.pm: Fix typo in device
  XML formatting. Allow pool source format to be specified
* scripts/storage/110-disk-pool.t: Test which creates a storage
  pool, creates some volumes, then deletes it all again.