virtio: console as a config option

I also added a small Kconfig change that allows the user to specify the
virtio console in menuconfig.

(Fixes to export symbols from Stephen Rothwell <sfr@canb.auug.org.au>)
(Fixes for CONFIG_VIRTIO_CONSOLE=y vs CONFIG_VIRTIO=m from Christian himself)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index d825361..6c070dc 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -624,8 +624,12 @@
 	  Xen virtual console device driver
 
 config VIRTIO_CONSOLE
-	bool
+	tristate "Virtio console"
+	depends on VIRTIO
 	select HVC_DRIVER
+	help
+	  Virtio console for use with lguest and other hypervisors.
+
 
 config HVCS
 	tristate "IBM Hypervisor Virtual Console Server support"