xenlight: only build xenlight if xen 4.4 or xen 4.5

The stubs need at least xen 4.4; previous versions appear to be
incompatible.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #58 from djs55/xl-for-4.4-only

xenlight: only build xenlight if xen 4.4 or xen 4.5