snapshot_edit: Add support to handle create snapshot without description

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.

Signed-off-by: Wayne Sun <gsun@redhat.com>
snapshot_edit: 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>
Merge pull request #236 from waynesun09/snapshot_edit

snapshot_edit: Add without_desc and delete_desc cases