Add vchan.xen, with a handy instantiation of the function

Inside vchan.xen we have

  module Vchan_xen: Vchan.S.ENDPOINT

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: install mirage-xen

This will trigger the conditional build.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #44 from djs55/add-xen

Add vchan.xen, with a handy instantiation of the function