vl: export machine_init_done

We have that variable but not exported.  Export that so modules can have
a way to poke on whether machine init has finished.

Meanwhile, set that up even before calling the notifiers, so that
notifiers who may depend on this field will get a correct answer.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180306053320.15401-2-peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>