virt: Implement subdirectory flattening classes

* data_dir.SubdirList class returns a flat-list of all non-hidden
  subdirectories beneath a base directory (inclusive).

* data_dir.SubdirGlobList class returns flat-list of all files
  matching a glob pattern, within all non-hidden subdirectories
  beneath a base directory (inclusive).
Signed-off-by: NChris Evich <cevich@redhat.com>