Allow building against xen-4.2 as well as xen-4.3

This is useful because xenopsd now depends on the new xenlight ocaml interfaces, but xen-4.3 is not commonly found in distros yet (CentOS, Debian, Ubuntu) [although it's in Debian unstable].

The idea is to detect the xen-4.3 definitions and to guard uses with #defines. The only one that is possibly bad for xenopsd is the DOMAIN_TYPE?

I've tested this on xen-4.2 only, not yet xen-4.3