[Merge] lp:~paelzer/ubuntu-seeds/refactor-platform-virt-artful into lp:~ubuntu-core-dev/ubuntu-seeds/ubuntu.artful

[ChristianEhrhardt]

In testing with the combined change I see all references to the task drop correctly.
Files: virt-host.*
Further I see the replace-seeds for main show up in supported-misc-servers.seedtext
And finally I see virt-host also removed in structure.dot

Ready for review
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https://code.launchpad.net/~paelzer/ubuntu-seeds/refactor-platform-virt-artful/+merge/327076
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