Trivial test of a successful write.
Signed-off-by: NGreg Kurz <groug@kaod.org>
(groug, handle potential overflow when computing request size,
        add missing g_free(buf),
        backend handles one written byte at a time to validate
        the server doesn't do short-reads)
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>