fix: bring default valuesContext when customizing vm-import-controller addon (backport #1267)

[mergify]

Summary

fix: bring default this.value.spec.valuesContext when customizing vm-import controller

valuesContent: |
    image:
      tag: v0.4.1
    fullnameOverride: harvester-vm-import-controller

PR Checklist

• Is this a multi-tenancy feature/bug?
  • Yes, the relevant RBAC changes are at:
• Do we need to backport changes to the old Rancher UI, such as RKE1?
  • Yes, the relevant PR is at:
• Are backend engineers aware of UI changes?
  • Yes, the backend owner is: @ibrokethecloud

Related Issue #
harvester/harvester#7351 (comment)

Occurred changes and/or fixed issues

Will port to harvester-ui-extension once this PR merged.

Areas which could experience regressions

Screenshot/Video

fix_vm_import.mov

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This is an automatic backport of pull request #1267 done by [Mergify](https://mergify.com).