Allow building against xen-4.3

The xen-4.3 branch doesn't build against xen-4.3.1 installed on my Debian box. My guess is this code is designed to build against xen-4.4-- is this correct?

These patches attempt to detect the xen-4.4 definitions and guard them with #defines. This is useful because xen-4.3 is only starting to appear in distros (Debian unstable) and it'll be ages before xen-4.4 gets anywhere, since it has to be released first.

I've tested this on xen-4.3 only, not yet xen-4.4.