In Xen 4.2, xs.h is deprecated in favor of xenstore.h.  xs.h now
contains

#warning xs.h is deprecated use xenstore.h instead
#include <xenstore.h>

which fails compilation when warnings are treated as errors.

Introduce a configure-time check for xenstore.h and if found,
use it instead of xs.h.
(cherry picked from commit 416eca18)