tests: qemuxml2xml: Always use different output file
    
    Most qemuxml2xml tests expect that the input XML is unchanged after
    parsing. This is unlike 99% of new qemu configs in the wild, which after
    initial parsing end up with stable PCI device addresses. The xml2xml bit
    doesn't currently hit that code path though, so most XML testing indeed
    does not change.
    
    Future patches will add that PCI address bits, which means most test cases
    will have different output. So let's do away with the hardcoded same vs
    different test split, and always track a separate output file. Tests can
    still have same input and output, it just necessitates 2 separate XML files.