Add cloud-init example for guest DNS resolver configuration

[ndecandia]

What this PR does / why we need it:

This PR improves the DNS records documentation by adding a practical cloud-init example for configuring the guest DNS resolver.

The current documentation explains the requirement for options ndots:2 or higher when resolving short names such as myvmi.mysubdomain, but it does not provide an example on how to configure the guest operating system.

The added example demonstrates how to configure DNS settings using cloud-init in a persistent and systemd-resolved friendly way.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Fixes #

Special notes for your reviewer:

This is a documentation-only improvement.

Checklist

• Design: not required
• PR: The PR description is expressive enough and will help future contributors
• Code: not applicable
• Refactor: not applicable
• Upgrade: not applicable
• Testing: not applicable
• Documentation: user-guide update included
• Community: not required

Release note:

NONE

[kubevirt-bot]

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[kubevirt-bot]

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