stats: allow configuring poller delay

This introduces a startup config option for configuring stats poller delay.

Use `stats { interval <seconds> }` to configure the delay at startup.

The default value remains unchanged - 10 seconds.

Change-Id: If12cb1f7f6f1f8ecfa461561bc77847cdf260388
Signed-off-by: Klement Sekera <ksekera@cisco.com>
diff --git a/src/vnet/api_errno.h b/src/vnet/api_errno.h
index 8c509d7..f928e0d 100644
--- a/src/vnet/api_errno.h
+++ b/src/vnet/api_errno.h
@@ -15,6 +15,10 @@
 #ifndef included_vnet_api_errno_h
 #define included_vnet_api_errno_h
 
+#include <stdarg.h>
+#include <vppinfra/types.h>
+#include <vppinfra/format.h>
+
 #define foreach_vnet_api_error						\
 _(UNSPECIFIED, -1, "Unspecified Error")                                 \
 _(INVALID_SW_IF_INDEX, -2, "Invalid sw_if_index")                       \
@@ -147,6 +151,30 @@
     VNET_API_N_ERROR,
 } vnet_api_error_t;
 
+/* *INDENT-OFF* */
+static inline u8 *
+format_vnet_api_errno (u8 * s, va_list * args)
+{
+  vnet_api_error_t api_error = va_arg (*args, vnet_api_error_t);
+#ifdef _
+#undef _
+#endif
+#define _(a, b, c)           \
+  case b:                    \
+    s = format (s, "%s", c); \
+    break;
+  switch (api_error)
+    {
+      foreach_vnet_api_error
+      default:
+       	s = format (s, "UNKNOWN");
+        break;
+    }
+  return s;
+#undef _
+}
+/* *INDENT-ON* */
+
 #endif /* included_vnet_api_errno_h */
 
 /*