xen: return non-null for xs_daemon_open

Because we have nothing to open, we return nullptr in this compatibility
function. The problem, however, is that the libxenstore version of it will
return a handle, with nullptr meaning failure.

That leads to some breakages for code using libxenstore.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>