Add IsCVM value to KeywordName column in events

[maddieford]

Description

Currently, we send a single telemetry event on service initialization which indicates if the VM is a CVM or not.
As we prepare telemetry releases for CVM-only features, we will need to write queries that efficiently filter events for CVMs only. This PR adds the IsCVM key to the KeywordName column. The agent currently discovers if the VM is a CVM over a network call, so the first few events in agent initialization will not have the 'IsCVM' key.

Issue #

PR information

• Ensure development PR is based on the develop branch.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made

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Distro maintenance information, if applicable

• This is a contribution from a distro maintainer
• The changes in this PR have been taken as a downstream patch (Note: it is not recommended to patch the agent without upstream review and approval)

[nagworld9]

None works. But for explicit checking, like GUID_ZERO for container id, can we do similarly assign IsCVM_UNINITIALIZED as default here and identify same string in event.py?

[maddieford]

I thought about this, but I didn't like having string type as default value for bool value. For now I am keeping None but let me know if you have any concerns