migration: Wait on preempt channel in preempt thread

migration: Wait on preempt channel in preempt thread

QEMU main thread will wait until dest preempt channel established during
processing the LISTEN command (within the whole postcopy PACKAGED data), by
waiting on the semaphore postcopy_qemufile_dst_done.

That's racy, because it's possible that the dest QEMU main thread hasn't
yet accept()ed the new connection when processing the LISTEN event.  The
sem_wait() will yield the main thread without being able to run anything
else including the accept() of the new socket, which can cause deadlock
within the main thread.

To avoid the race, move the "wait channel" from main thread to the preempt
thread right at the start.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after main")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>