Fix broken specs in Kiwi::Image model

Two specs were failing when checking if the subject didn't have errors:

```
Failure/Error: it { expect(subject.errors).to be_empty }
  expected `[#<Nokogiri::XML::SyntaxError: 1:12: FATAL: XML declaration allowed only at the start of the document>].empty?` to be truthy, got false
./spec/models/kiwi/image_spec.rb:226:in `block (5 levels) in <top (required)>'

Failure/Error: expect(subject.errors).to be_empty
expected `[#<Nokogiri::XML::SyntaxError: 1:12: FATAL: XML declaration allowed only at the start of the document>].empty?` to be truthy, got false
./spec/models/kiwi/image_spec.rb:279:in `block (4 levels) in <top (required)>'
```

Those errors weren't reported before nokogiri _1.13.0_, as noted in the
changelog upstream:

> XML::DocumentFragment#errors now correctly contains any parsing errors
> encountered. Previously this was always empty. (Note that
> HTML::DocumentFragment#errors already did this.)

Relying on XML::DocumentFragment was wrong since the XML generated by
the Kiwi::Image's to_xml and kiwi_body methods is not a document
fragment, but a document. They contain a XML declaration (`<?xml ...?>`),
so changing the class solves the issue.