Better error message if xen-evtchn is not loaded

Fixes #13

Signed-off-by: David Scott <dave.scott@citrix.com>
Standardise on the ISC license

This one file had the old header.

Signed-off-by: David Scott <dave.scott@citrix.com>
Update CHANGES

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #15 from djs55/dependency

Better error message if xen-evtchn is not loaded