Fix block statistics with newer versions of Xen

Apparently the xen block device statistics moved from
"/sys/devices/xen-backend/vbd-%d-%d/statistics/%s"
to
"/sys/bus/xen-backend/devices/vbd-%d-%d/statistics/%s"

* src/xen/block_stats.c: try the extra path in case of failure to
  find the statistics in /sys

src/xen/block_stats.c

@@ -117,6 +117,18 @@ read_bd_stat (int device, int domid, const char *str)
     char path[PATH_MAX];
     int64_t r;
 
+    snprintf (path, sizeof path,
+              "/sys/bus/xen-backend/devices/vbd-%d-%d/statistics/%s",
+              domid, device, str);
+    r = read_stat (path);
+    if (r >= 0) return r;
+
+    snprintf (path, sizeof path,
+              "/sys/bus/xen-backend/devices/tap-%d-%d/statistics/%s",
+              domid, device, str);
+    r = read_stat (path);
+    if (r >= 0) return r;
+
     snprintf (path, sizeof path,
               "/sys/devices/xen-backend/vbd-%d-%d/statistics/%s",
               domid, device, str);