libxl: convert libxl__device_model_xs_path to a macro

libxl: convert libxl__device_model_xs_path to a macro

Since it's unsafe to code it as a function because it would end up passing a
non literal string to a printf like function.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>