Splitting into two functions allows the user to call the right
function, rather than having to remember that a *Free function is
an exception to the rule.

* src/conf/storage_conf.h (virStoragePoolSourceClear): New function.
* src/libvirt_private.syms (storage_conf.h): Export it.
* src/conf/storage_conf.c (virStoragePoolSourceFree): Split...
(virStoragePoolSourceClear): ...into new function.
(virStoragePoolDefFree, virStoragePoolDefParseSourceString):
Update callers.
* src/test/test_driver.c (testStorageFindPoolSources): Likewise.
* src/storage/storage_backend_fs.c
(virStorageBackendFileSystemNetFindPoolSourcesFunc)
(virStorageBackendFileSystemNetFindPoolSources): Likewise.
* src/storage/storage_backend_iscsi.c
(virStorageBackendISCSIFindPoolSources): Likewise.
* src/storage/storage_backend_logical.c
(virStorageBackendLogicalFindPoolSources): Likewise.