migration: fix migrate_cancel leads live_migration thread hung forever

migration: fix migrate_cancel leads live_migration thread hung forever

When we 'migrate_cancel' a multifd migration, live_migration thread may
hung forever at some points, because of multifd_send_thread has already
exit for socket error:
1. multifd_send_pages may hung at qemu_sem_wait(&multifd_send_state->
   channels_ready)
2. multifd_send_sync_main my hung at qemu_sem_wait(&multifd_send_state->
   sem_sync)

Signed-off-by: Ivan Ren <ivanren@tencent.com>
Message-Id: <1561468699-9819-3-git-send-email-ivanren@tencent.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
---

Remove spurious not needed bits