evacuate a vm when it is evacuating will cause vm error

[efineshi]

Description
===========

if we evacuate a vm agent when if is still evacuating, it will leads the vm be many unexpect errors.

so, wo don't allow the vm to be evacuated when it is in rebuilding state before Train version, but this patch makes it allowed.

https://review.opendev.org/c/openstack/nova/+/864169

Steps to reproduce
==================

1. create a vm

2. down the host compute node

3. evacuate the vm last twice

openstack server evacuate XXXXX
openstack server evacuate XXXX

Expected result
===============

it should be not allowed to evacuate twice.

Actual result
=============

the 2nd evacuate are allowed and the vm will be error, if the vm is created in the other node, but not

evacuate completed, it will be evacuate again, and leads to many errors, such as double alive, or if it

is schedued to the same node, it will be error and can't be evacuate as the vm or vm files exsit.

Environment
===========

openstack wallaby / master

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/898200