migration/postcopy: ensure preempt channel is ready before loading states

migration/postcopy: ensure preempt channel is ready before loading states

Before loading the guest states, ensure that the preempt channel has been
ready to use, as some of the states (e.g. via virtio_load) might trigger
page faults that will be handled through the preempt channel. So yield to
the main thread in the case that the channel create event hasn't been
dispatched.

Cc: qemu-stable <qemu-stable@nongnu.org>
Fixes: 9358982744 ("migration: Send requested page directly in rp-return thread")
Originally-by: Lei Wang <lei4.wang@intel.com>
Link: https://lore.kernel.org/all/9aa5d1be-7801-40dd-83fd-f7e041ced249@intel.com/T/
Signed-off-by: Lei Wang <lei4.wang@intel.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Link: https://lore.kernel.org/r/20240405034056.23933-1-wei.w.wang@intel.com
[peterx: add a todo section, add Fixes and copy stable for 8.0+]
Signed-off-by: Peter Xu <peterx@redhat.com>