build-flavors: T6913: fix Xen guest agent package in the generic image

[dmbaturin]

Change Summary

1. Use the new xen-guest-agent package in the generic image, instead of the old vyos-xe-guest-utilities that's no longer there.
2. Remove the old XCP-NG flavor because the new agent works for both mainline Xen and XCP-NG, so there's no need for separate mainline xe-guest-utilities and the vyos-xe-guest-utilities fork anymore.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build flavors.

How to test

sudo ./build-vyos-image generic should succeed and the image should work in XCP-NG and other Xen systems as expected.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

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