xen: use the hvc console infrastructure for Xen console

Implement a Xen back-end for hvc console.

* * *
Add early printk support via hvc console, enable using
"earlyprintk=xen" on the kernel command line.

From: Gerd Hoffmann <kraxel@suse.de>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Olof Johansson <olof@lixom.net>

include/xen/events.h

@@ -13,6 +13,7 @@ enum ipi_vector {
 	XEN_NR_IPIS,
 };
 
+int bind_evtchn_to_irq(unsigned int evtchn);
 int bind_evtchn_to_irqhandler(unsigned int evtchn,
 			      irq_handler_t handler,
 			      unsigned long irqflags, const char *devname,

include/xen/hvc-console.h

@@ -0,0 +1,6 @@
+#ifndef XEN_HVC_CONSOLE_H
+#define XEN_HVC_CONSOLE_H
+
+extern struct console xenboot_console;
+
+#endif	/* XEN_HVC_CONSOLE_H */