Fix virtual grains for VMs running on Nutanix AHV

[meaksh]

What does this PR do?

This PR fixes virtual grains detections for instances running on Nutanix AHV.

I took the approach discussed at this old unmerged PR: #62387

What issues does this PR fix or reference?

Fixes #67180

Previous Behavior

# salt-call grains.items
[...]
virtual:
    physical

New Behavior

# salt-call grains.items
[...]
virtual:
    kvm
virtual_subtype:
    Nutanix AHV

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

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