CP-4405: Pull relevant commits from xen-api-libs

Pull in any relevant commits to xen-api-libs that have happened since xen-api-libs-transitional branched.

Deliberately excluded https://github.com/xapi-project/xen-api-libs/commit/dcadccff0ffb71e09ad0583d3269d37de5477ad4 since this change is to be made in ocaml-xen-lowlevel-libs: https://github.com/xapi-project/ocaml-xen-lowlevel-libs/pull/27