util: xml: Expose all arguments of virXMLParseHelper in virXMLParse macro

util: xml: Expose all arguments of virXMLParseHelper in virXMLParse macro

The generic helper also has helper code to validate the root element and
create an XPath context. Many places in the code duplicate code for
doing these operations.

Extend the helper to provide all arguments and fix all callers.

In many cases this patch refactors the passing of the 'validate'
field into a separate variable to avoid quirky looking arguments.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>