kvm: Fix test for boot images

The test have incorrect leftover of old limited implementation, now the system
shall update images by itself.

Also the typical scenario - image on first request - was not covered.

Adapt the test to code without needed linking.
9 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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