While backup depends on an encrypted volume type this type should not be removed

[Yao wang]

While backup depends on an encrypted volume type this type should not be removed

How to reproduce:
- Create an encryption volume type
- Use this type to create an encryption volume
- Create a volume backup using from this encryption volume
- Delete the encryption volume
- Delete the encryption volume type
- Your backup cannot be used in any way

Suppose I have backed up an encrypted volume, If the encryption type is deleted while this backup exists, the backup will be permanently unusable, which is unreasonable.
I think it is necessary to check if there is a backup dependency before deleting the encryption type.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/734427

[Yao wang (wangyao18322710251)]

Encryption volume type may be changed,so this code will not solve the problem https://review.opendev.org/734427

The only way I can think of is to iterate over all the base metadata of the Backup Driver. I can't understand why not add a volume_type_id field on table 'backups'.

Do you have any Suggestions?