In migration_incoming_state_destroy(), we've got a few variables that aren't
destroyed properly, namely:

    main_thread_load_event
    postcopy_pause_sem_dst
    postcopy_pause_sem_fault
    rp_mutex

Destroy them properly.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20200908203022.341615-2-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>