xen: xenbus_dev_frontend: Really return response string

xenbus_command_reply() did not actually copy the response string and
leaked stack content instead.

Fixes: 9a6161fe ("xen: return xenstore command failures via response instead of rc")
Signed-off-by: Simon Gaiser <simon@invisiblethingslab.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>