[xs64bit] CP-8987: Support extensible Xenops suspend image records

Backport of https://github.com/xapi-project/xenopsd/pull/87 where possible. Note that the backport has been done in a compatible way with upstream xenopsd in https://github.com/xapi-project/xenopsd/pull/87 but without the use of sexplib.

This has been tested by migrating from a Creedence build (with this xenopsd) to a trunk xenopsd (which has already been extended to include the serialised VM Xenopsd record)