fixes #343: new data-source for ovirt_disk_attachments

[engelmi]

Please describe the change you are making

This PR fixes #343 and adds the ability to retrieve disk attachments by VM ID.

Are you the owner of the code you are sending in, or do you have permission of the owner?

yes

The code will be published under the BSD 3 clause license. Have you read and understood this license?

yes

[ovirt-infra]

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Very good code, please run go generate and verify the docs.