tests, utils: Use `cdi.kubevirt.io/v1beta1` for datavolume
tests, utils: Use `cdi.kubevirt.io/v1beta1` for datavolume

The `v1beta1` API version is available for some time now [1].
Use it instead of the `alpha` one and inline the constant as it is used
only once (there is also no added value in naming it using a constant).

[1] https://github.com/kubevirt/containerized-data-importer/pull/1232

Signed-off-by: Edward Haas <416ab6a6c3e37a8cab0d365516e79cce8f9be5ad@redhat.com>

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