plugins/xml-pack: Only allow formatting XML

JCWasmx86 requested to merge JCWasmx86/gnome-builder:xmlfmt_fix into main

HTML won't work, as libxml adds some XML header in the first line.

Furthermore use xmlIndentTreeOutput and xmlKeepBlanksDefault to fix some formatting issues.

I'm not sure there is a better way, as the API documentation is imho quite sparse and the API itself is really arcane.

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