Storage pool duplicate UUID/name checking

Storage pool duplicate UUID/name checking

The storage pool driver is not doing correct checking for
duplicate UUID/name values. This introduces a new method
virStoragePoolObjIsDuplicate, based on the previously
written virDomainObjIsDuplicate.

* src/conf/storage_conf.c, src/conf/storage_conf.c,
  src/libvirt_private.syms: Add virStoragePoolObjIsDuplicate,
* src/storage/storage_driver.c: Call virStoragePoolObjIsDuplicate
  for checking uniqueness of uuid/names