migration/postcopy: Replace QemuSemaphore with QemuEvent

thread_sync_sem is an one-shot event so it can be converted into
QemuEvent, which is more lightweight.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250529-event-v5-9-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
3 files changed