CA-100704: Fix xenguest

From http://ocamlunix.forge.ocamlcore.org/ocamlunix.html

The flag list must contain exactly one of the following flags:
O_RDONLY	Open in read-only mode.
O_WRONLY	Open in write-only mode.
O_RDWR		Open in read and write mode.

So I've added exactly one of those flags!

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>