Update tagging logic in volume attachments in is_instance

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

=== RUN   TestAccIBMISInstanceVolumeAttachment_userTag
--- PASS: TestAccIBMISInstanceVolumeAttachment_userTag (302.51s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     306.293s