Virtio console driver

This is an hvc-based virtio console driver.  It's suboptimal becuase
hvc expects to have raw access to interrupts and virtio doesn't assume
that, so it currently polls.

There are two solutions: expose hvc's "kick" interface, or wean off hvc.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_console.h

@@ -0,0 +1,12 @@
+#ifndef _LINUX_VIRTIO_CONSOLE_H
+#define _LINUX_VIRTIO_CONSOLE_H
+#include <linux/virtio_config.h>
+
+/* The ID for virtio console */
+#define VIRTIO_ID_CONSOLE	3
+
+#ifdef __KERNEL__
+int __init virtio_cons_early_init(int (*put_chars)(u32, const char *, int));
+#endif /* __KERNEL__ */
+
+#endif /* _LINUX_VIRTIO_CONSOLE_H */