Update README.md

[ianb-mp]

Clarify the 'Content Types' section. Add a reference to Kubevirt filesystem feature (virtiofs).

Relates to #3168

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[awels]

/lgtm
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[awels]

/release-note-none

[awels]

/test pull-containerized-data-importer-e2e-hpp-latest

[porjo]

/retest

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@porjo: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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[akalenyu]

/test pull-containerized-data-importer-e2e-hpp-latest