Allow performing a linked-clone from a template without requiring snapshots

[arizvisa]

Description

This PR allows performing a linked clone from a template by using the "moveAllDiskBackingAndAllowSharing" option. When performing a clone, the source virtual machine is checked if it's marked as a template. If so, then it sets the option and proceeds with the clone. If it is not marked as a template, then it will proceed with the original logic which validates that a snapshot exists.

The documentation was updated to remove reference to the snapshot constraint.

Acceptance tests

There were no tests for the cloning functionality, and so I wasn't sure how to proceed here.

• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch? (If so, please include the test log in a gist)

References

This closes issue #1156.

[arizvisa]

Added another commit to handle a case in ValidateVirtualMachineClone where I wasn't validating the machine being a template before it called validateCloneSnapshots. This wasn't caught before because I was initially using commit 7e3c4f38186fd7a4299658def6d9d9ac9dde0d8an (first one in my PR) in which I patched a way to break out of validateCloneSnapshots upon identification of the template flag.

Now, instead of tampering with the logic of validateCloneSnapshots, machines are checked for the template marking before it actually being called. This might help with the implementation of the solution to issue #831 where this function can instead be rewritten to validate a particular snapshot identifier and use that one.

[bill-rich]

Thanks for fixing this case. I'm going to be adding more power state management for VMs in the near future, so templates managed by the provider will become more useful shortly. This is a really good step toward that.

[ghost]

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