vty: use internal node id for xml ids

The 'show online-help' produces XML output with <node id="..."> ids.  We
reference those from the osmo-gsm-manuals, but until now, these ids fall out of
sync when the amount of VTY nodes changes.

Change these ids to use the internal node ID constant (as in enum bsc_vty_node)
instead of a simple counter.

If this is merged, we need to adjust the references in osmo-gsm-manuals.git.

Change-Id: Ib07fb9d9106e19f5be6539493e82b5d5991f8bc2
diff --git a/src/vty/command.c b/src/vty/command.c
index a19d5db..af936ee 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -617,7 +617,7 @@
 		if (!cnode)
 			continue;
 
-		vty_out(vty, "  <node id='%d'>%s", i, VTY_NEWLINE);
+		vty_out(vty, "  <node id='%d'>%s", cnode->node, VTY_NEWLINE);
 
 		for (j = 0; j < vector_active(cnode->cmd_vector); ++j) {
 			struct cmd_element *elem;