vl: export machine_init_done

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 <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20180306053320[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>