xencons: further Dom0 handling improvements

c/s 1242:731ff1f10c46 ("xencons: generalize use of
add_preferred_console()") still left cases where (in Dom0) the console
would get registered with index -1. Eliminate these cases by
- also calling add_preferred_console() in Dom0 when "xencons=" was
  specified
- setting the index directly when in Dom0 and "xencons=" was not given

Also do some cleanup:
- Move the declaration of console_use_vt into the respective global
  header (where it should have been placed from the beginning), and
  use a #define instead of a variable when !XEN.
- Replace the needless uses of goto in xen_console_init() with plain
  return statements.

Reported-by: Olaf Hering <olaf@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -130,7 +130,9 @@ LIST_HEAD(tty_drivers);			/* linked list
    vt.c for deeply disgusting hack reasons */
 DEFINE_MUTEX(tty_mutex);
 
+#ifndef console_use_vt
 int console_use_vt = 1;
+#endif
 
 #ifdef CONFIG_UNIX98_PTYS
 extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
--- a/drivers/xen/console/console.c
+++ b/drivers/xen/console/console.c
@@ -86,9 +86,8 @@ static int __init xencons_setup(char *st
 {
 	char *q;
 	int n;
-	extern int console_use_vt;
 
-	console_use_vt = 1;
+	console_use_vt = -1;
 	if (!strncmp(str, "ttyS", 4)) {
 		xc_mode = XC_SERIAL;
 		str += 4;
@@ -193,13 +192,13 @@ static struct console kcons_info = {
 static int __init xen_console_init(void)
 {
 	if (!is_running_on_xen())
-		goto out;
+		return 0;
 
 	if (is_initial_xendomain()) {
 		kcons_info.write = kcons_write_dom0;
 	} else {
 		if (!xen_start_info->console.domU.evtchn)
-			goto out;
+			return 0;
 		kcons_info.write = kcons_write;
 	}
 
@@ -229,16 +228,17 @@ static int __init xen_console_init(void)
 		break;
 
 	default:
-		goto out;
+		return 0;
 	}
 
 	wbuf = alloc_bootmem(wbuf_size);
 
-	if (!is_initial_xendomain())
+	if (console_use_vt <= 0 || !is_initial_xendomain())
 		add_preferred_console(kcons_info.name, xc_num, NULL);
+	else
+		kcons_info.index = xc_num;
 	register_console(&kcons_info);
 
- out:
 	return 0;
 }
 console_initcall(xen_console_init);
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -63,6 +63,12 @@ extern const struct consw vga_con;	/* VG
 extern const struct consw newport_con;	/* SGI Newport console  */
 extern const struct consw prom_con;	/* SPARC PROM console */
 
+#ifdef CONFIG_XEN
+extern int console_use_vt;
+#else
+#define console_use_vt 1
+#endif
+
 int con_is_bound(const struct consw *csw);
 int register_con_driver(const struct consw *csw, int first, int last);
 int unregister_con_driver(const struct consw *csw);