BSD: implement nodeGetCPUStats

Implementation obtains CPU usage information using
kern.cp_time and kern.cp_times sysctl(8)s and reports
CPU utilization.

include/libvirt/libvirt.h.in

@@ -691,6 +691,14 @@ typedef enum {
  */
 #define VIR_NODE_CPU_STATS_IOWAIT "iowait"
 
+/**
+ * VIR_NODE_CPU_STATS_INTR:
+ *
+ * The cumulative interrupt CPU time,
+ * since the node booting up (in nanoseconds).
+ */
+#define VIR_NODE_CPU_STATS_INTR "intr"
+
 /**
  * VIR_NODE_CPU_STATS_UTILIZATION:
  *