make xen ocaml safe-strings compliant

make xen ocaml safe-strings compliant

Xen built with ocaml 4.06 gives errors such as
Error: This expression has type bytes but an expression was
        expected of type string
as Byte and safe-strings which were introduced in 4.02 are the
default in 4.06.
This patch which is partly by Richard W.M. Jones of Red Hat
from https://bugzilla.redhat.com/show_bug.cgi?id=1526703
fixes these issues.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Christian Lindig<christian.lindig@citrix.com>