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: Chris Evich <cevich@redhat.com>
virt: Support test module arbitrary organization

Signed-off-by: Chris Evich <cevich@redhat.com>
virt: Support test config. arbitrary orginization

Signed-off-by: Chris Evich <cevich@redhat.com>
virt: Some minor style/pylint cleanup

Signed-off-by: Chris Evich <cevich@redhat.com>