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 mostly by Richard W.M. Jones of Red Hat
from https://bugzilla.redhat.com/show_bug.cgi?id=1526703
fixes these issues.

v2: drop tools/ocaml/libs/xc/xenctrl.ml from the patch as the
affected code was removed by commit d933f1a53c06002351c1e36d40615e40bd4bf6af
tools/ocaml: Drop coredump infrastructure

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>
[ wei: remove trailing whitespaces ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>