Rearrange snapshot updating SMAPI code

- Implement SR.update_snapshot_info_dest
- Call the above from SR.update_snapshot_info_src
- Remove VDI.set_snapshot_of, VDI.set_snapshot_time,
  VDI.set_is_a_snapshot from SMAPI

Signed-off-by: John Else <john.else@citrix.com>