conf: storage: Add volume feature formatter for gluster pools

Libvirt didn't output feature flags for images stored on native gluster.
Fix this trivially by adding a feature formatter callback.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1095035

src/conf/storage_conf.c

@@ -258,6 +258,8 @@ static virStoragePoolTypeInfo poolTypeInfo[] = {
          .defaultFormat = VIR_STORAGE_FILE_RAW,
          .formatToString = virStorageFileFormatTypeToString,
          .formatFromString = virStorageVolumeFormatFromString,
+         .featureFromString = virStorageFileFeatureTypeFromString,
+         .featureToString = virStorageFileFeatureTypeToString,
      }
     },
     {.poolType = VIR_STORAGE_POOL_MPATH,