storage: improve directory support in gluster pool

storage: improve directory support in gluster pool

Take advantage of the previous patch's addition of 'netdir' as
a distinct volume type, to expose rather than silently skip
directories embedded in a gluster pool.  Also serves as an XML
validation for the previous patch.

* src/storage/storage_backend_gluster.c
(virStorageBackendGlusterRefreshVol): Don't skip directories.
* tests/storagevolxml2xmltest.c (mymain): Add test.
* tests/storagevolxml2xmlin/vol-gluster-dir.xml: New file.
* tests/storagevolxml2xmlout/vol-gluster-dir.xml: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>