mirage-xen.0.9.4 and mirage-xen.0.9.5 need an exact versions of xenstore

Fixes mirage/ocaml-xen-block-driver#2

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #1047 from djs55/mirage-xen-versions

mirage-xen.0.9.4 and mirage-xen.0.9.5 need an exact versions of xenstore