Fix compilation of legacy xen driver with Xen 4.2

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)

configure.ac

@@ -691,6 +691,8 @@ if test "$with_xen" != "no" ; then
 fi
 
 if test "$with_xen" != "no" ; then
+    dnl In Xen 4.2, xs.h is deprecated in favor of xenstore.h.
+    AC_CHECK_HEADERS([xenstore.h])
     AC_CHECK_HEADERS([xen/xen.h xen/version.h xen/dom0_ops.h],,[
        if test "$with_xen" = "yes"; then
            fail=1

src/xen/block_stats.c

@@ -20,7 +20,11 @@
 # include <unistd.h>
 # include <regex.h>
 
-# include <xs.h>
+# if HAVE_XENSTORE_H
+#  include <xenstore.h>
+# else
+#  include <xs.h>
+# endif
 
 # include "virterror_internal.h"
 # include "datatypes.h"

src/xen/xs_internal.c

@@ -23,7 +23,11 @@
 #include <xen/dom0_ops.h>
 #include <xen/version.h>
 
-#include <xs.h>
+#if HAVE_XENSTORE_H
+# include <xenstore.h>
+#else
+# include <xs.h>
+#endif
 
 #include "virterror_internal.h"
 #include "datatypes.h"