Move oval_results_model_export_source to public API.
Refactor: Extract function: lookup_xsi_ns.
Do not export namespaced property by xmlNewPror

Use xmlNewNsProp instead.

Addressing (pre-export validation error):
OpenSCAP Error: File '/tmp/oscap.HceiZV/stub-oval.xml.result.xml' line 0:
	Element '{http://oval.mitre.org/XMLSchema/oval-results-5}oval_results',
	attribute 'xsi:schemaLocation': The attribute 'xsi:schemaLocation' is not allowed.
		[xccdf_session.c:346]
Could not export OVAL Results correctly to /tmp/oscap.HceiZV/stub-oval.xml.result.xml [xccdf_session.c:1202]
Drop OVAL_XMLNS_XSI.
Drop XCCDF_XSI_NAMESPACE.
tests: namespaces gets exported before the properties

Amend the exemplary content.
tests: XSI namespace is now exported only once

Amend the exemplary content.