virt tests: cleanup all remaining disk images

[cbosdo]

What does this PR change?

Some disks images are left over in the default pool after the KVM or Xen tests. Ensure those are removed at the end of the test.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: test suite enhancement

• DONE

Test coverage

• No tests: tests fix

• DONE

Links

• DONE

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[cbosdo]

gitarro no changelog needed !!!

[Bischoff]

LGTM, but see nitpicks.