windowsaio: initialize and map windowsaio IO structure to io_u

Instead of searching for a free entry everytime we enter
queue, do this when the io_u is setup. It's cleaner and
faster.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/ioengine.h b/ioengine.h
index 997f90a..0285b08 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -121,6 +121,8 @@
 	int (*close_file)(struct thread_data *, struct fio_file *);
 	int (*get_file_size)(struct thread_data *, struct fio_file *);
 	void (*terminate)(struct thread_data *);
+	int (*io_u_init)(struct thread_data *, struct io_u *);
+	void (*io_u_free)(struct thread_data *, struct io_u *);
 	int option_struct_size;
 	struct fio_option *options;
 	void *data;