Add `with sexp` to the Lv types.
Add `with sexp` to the Lv types.

This allows easy marshalling of (e.g.) Lv Segment.t lists.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

djs55/mirage-block-volume,simonjbeaumont/mirage-block-volume