Correct the polarity of ring pointers in unix impl

The unix implementation's xs_transport_xen module's read and write functions were using the "frontend" view of the ring. But this code is only ever used by xenstored in dom0, which should always have the "backend" view of the ring.

The symptom of this was that the "req" and "rsp" pointers were inverted.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>