vty: Implement missing public API host_config_file()

The API was defined in the header file but not implemented.

Change-Id: I69de9864fa7cd421557b9fc630beefea6d90b4cc
diff --git a/src/vty/command.c b/src/vty/command.c
index 4825109..6d63d84 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -4219,6 +4219,11 @@
 	host.config = talloc_strdup(tall_vty_cmd_ctx, filename);
 }
 
+const char *host_config_file(void)
+{
+	return host.config;
+}
+
 /*! Deprecated, now happens implicitly when calling install_node().
  * Users of the API may still attempt to call this function, hence
  * leave it here as a no-op. */