tests: qemuxml2xml: Format status XML header dynamically

tests: qemuxml2xml: Format status XML header dynamically

Status XML tests were done by prepending a constant string to an
existing XML. With the planned changes the header will depend on data
present in the definition rather than just on the data that was parsed.

The first dynamic element in the header will be the vcpu thread list.
Reuse and rename qemuXML2XMLPreFormatCallback for gathering the relevant
data when checking the active XML parsing and formating and pass the
bitmap to a newly crated header generator.