fix dependency of mirage-xen on OCaml versions
All recent versions of mirage-xen have a configure script that mandates OCaml version 4.00.1 or 4.01.0
mirage-xen: more constraints on OCaml versions
Apply the constraints in mirage-xen.0.9.3 to 0.9.1 and 0.9.2
Merge pull request #2984 from samoht/mirage-xen

Mirage xen