lguest: explicitly set miscdevice's private_data NULL

There is a proposed change to the miscdevice's behaviour on open(). Currently
file->private_data stays NULL, but only because we don't have an open-entry in
struct file_operations.

This may change so that private_data, more consistently, is always set to
struct miscdevice, not only *if* the driver has it's own open() routine and
fops-entry, see https://lkml.org/lkml/2014/12/4/939

 and commit
94e4fe2c

In short: If we rely on file->private_data being NULL, we should ensure
it is NULL ourselves.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>