Description of problem:

When there are multiple VSLs in a cluster with different provider types, and none of them is set as default, Velero results in a validation error, making migrations fail. 

```
status:
  expiration: '2023-04-10T05:45:09Z'
  formatVersion: 1.1.0
  phase: FailedValidation
  validationErrors:
    - >-
      provider aws has more than one possible volume snapshot location, and none
      were specified explicitly or as a default
  version: 1
```

```
status:
  conditions:
    - category: Advisory
      durable: true
      lastTransitionTime: '2023-03-21T15:45:58Z'
      message: 'The migration has failed.  See: Errors.'
      reason: InitialBackupCreated
      status: 'True'
      type: Failed
  errors:
    - >-
      provider aws has more than one possible volume snapshot location, and none
      were specified explicitly or as a default
    - >-
      Backup openshift-migration/aiocptools-portal-cutover-2-initial-rll9b:
      validation failed.
```

Version-Release number of selected component (if applicable):
1.7.8

How reproducible:
Always, apparently.

Steps to Reproduce:
This is purely from what I understood communicating with the customer.

1. Create 2 migstorages of GCP provider, create another 2 with AWS provider.
2. Run a migration selecting any one of the MigStorages. 

Actual results:
Migration fails with above error.

Expected results:
Migration should succeed.