vmstat: Add root element to libxo output

Current libxo output does not have a root element. Valid XML requires a single
root element. This commit adds this root element.

The libxo output version bumped accordingly.

PR:		254635
Pull Request: #1330

(cherry picked from commit c7dd97e)

usr.bin/vmstat/vmstat.c

@@ -79,7 +79,7 @@ static char sccsid[] = "@(#)vmstat.c	8.1 (Berkeley) 6/6/93";
 #include <libutil.h>
 #include <libxo/xo.h>
 
-#define VMSTAT_XO_VERSION "1"
+#define VMSTAT_XO_VERSION "2"
 
 static char da[] = "da";
 
@@ -295,6 +295,7 @@ main(int argc, char *argv[])
 	argv += optind;
 
 	xo_set_version(VMSTAT_XO_VERSION);
+	xo_open_container("vmstat");
 	if (!hflag)
 		xo_set_options(NULL, "no-humanize");
 	if (todo == 0)
@@ -396,6 +397,7 @@ main(int argc, char *argv[])
 		dointr(interval, reps);
 	if (todo & VMSTAT)
 		dovmstat(interval, reps);
+	xo_close_container("vmstat");
 	xo_finish();
 	exit(0);
 }