Add support for multiple disks to testcloud plugin

[happz]

TMT_SHOW_TRACEBACK=1 tmt -vv run plans --default \
    provision --how virtual --image fedora-39 \
              --hardware 'disk[1].size=20GB' \
              --hardware 'disk[0].size=15GB' \
    login -s provision \
    finish

Fixes https://github.com/teemtee/tmt/issues/2765

Pull Request Checklist

• [x] implement the feature
• [ ] extend the test coverage
• [ ] mention the version
• [ ] include a release note