Previousxenversion

Hello,
for version previous than 4.4 (I've tested only to the 4.2.1 at the moment) the xl info command will return something like this:
xen_major              : 4
xen_minor              : 2
xen_extra              : .1

So the script in the Makefile will always fail. This commit fix the build script for older XEN version too.

Attached a small patch that should work for previous XEN version