Sometimes, we destroy the dirty_bitmap in kvm_memory_slot before any sync action
occur, this bit in dirty_bitmap will be missed, and which will lead the corresponding
dirty pages to be missed in migration.

This usually happens when do migration during VM's Start-up or Reboot.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
[Use s->migration_log instead of exec.c's in_migration. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>