aio: fix qemu_bh_schedule() bh->ctx race condition

aio: fix qemu_bh_schedule() bh->ctx race condition

qemu_bh_schedule() is supposed to be thread-safe at least the first time
it is called.  Unfortunately this is not quite true:

  bh->scheduled = 1;
  aio_notify(bh->ctx);

Since another thread may run the BH callback once it has been scheduled,
there is a race condition if the callback frees the BH before
aio_notify(bh->ctx) has a chance to run.

Reported-by: Stefan Priebe <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Tested-by: Stefan Priebe <[email protected]>