qemu: refactor qemuBuildControllerDevStr to eliminate future duplicate code
    
    The PCI case of the switch statement in this function contains another
    switch statement with a case for each model. Currently every model
    except pci-root and pcie-root has a check for index > 0 (since only
    those two can have index==0), and the function should never be called
    for those two anyway. If we move the check for !pci[e]-root to the top
    of the pci case, then we can move the check for index > 0 out of the
    individual model cases. This will save repeating that check for the
    three new controller models about to be added.