tools: Introduce a non-truncating xc_xenver_changeset()

Update libxl and the ocaml stubs to match.  No API/ABI change in either.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>

tools/include/xenctrl.h

@@ -1579,6 +1579,7 @@ int xc_version(xc_interface *xch, int cmd, void *arg);
  */
 char *xc_xenver_extraversion(xc_interface *xch);
 char *xc_xenver_capabilities(xc_interface *xch);
+char *xc_xenver_changeset(xc_interface *xch);
 
 int xc_flask_op(xc_interface *xch, xen_flask_op_t *op);