snapshot_edit: Add without_desc and delete_desc cases

Add new filter case "without_desc" under disk_snapshot, update edit cmd
and snap_xml_compare function to fit the change. 

Update using CmdResult according to the snapshot_current function change.

Add delete_desc cases, the delete_desc cases is for delete description after
snapshot created with description. Update snap_xml_compare function for
check clone and rename cases.

Signed-off-by: Wayne Sun <gsun@redhat.com>