Fix _path used by tcm_fc

tcm_fc has a different name than the configfs dir it uses. When converting
to classes, we kept setting self.configfs_group but stopped overriding it
for tcm_fc, causing things to break.

Change docs to tell subclasses to override self._path, and fix fcoe class
to match.

Reported-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
update version to 2.1.39

Signed-off-by: Andy Grover <agrover@redhat.com>