CP-5913: Move users of "xs.h" to "xenstore.h"

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #74 on xapi-project/sm