Create a tag string with the given 'attributes' and parse that with libxml2.
This isn't quite correct to spec, but the best we can do here.

==26277== Conditional jump or move depends on uninitialised value(s)
==26277==    at 0x40375DB: getAtts (rsvg-css.c:1031)
==26277==    by 0x403905F: rsvg_css_parse_xml_attribute_string (rsvg-css.c:1097)
==26277==    by 0x4057273: rsvg_processing_instruction (rsvg-base.c:879)
==26277==    by 0x4BF8795: xmlParsePI (parser.c:5025)

https://bugzilla.gnome.org/show_bug.cgi?id=621636