virtio: console: Add file operations to ports for open/read/write/poll

Allow guest userspace applications to open, read from, write to, poll
the ports via the char dev interface.

When a port gets opened, a notification is sent to the host via a
control message indicating a connection has been established. Similarly,
on closing of the port, a notification is sent indicating disconnection.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_console.h

@@ -39,6 +39,7 @@ struct virtio_console_control {
 #define VIRTIO_CONSOLE_PORT_READY	0
 #define VIRTIO_CONSOLE_CONSOLE_PORT	1
 #define VIRTIO_CONSOLE_RESIZE		2
+#define VIRTIO_CONSOLE_PORT_OPEN	3
 
 #ifdef __KERNEL__
 int __init virtio_cons_early_init(int (*put_chars)(u32, const char *, int));