qemu: Fix error message when PCI bridge has index <= bus
    
    Commit ff2126225df0 changed the error message to be more
    detailed about the failure at hand; however, while the new
    error message claims that "bus must be <= index", the error
    message is displayed if "idx <= addr->bus", ie. when bus
    is larger than or *equal to* index.
    
    Change the error message to report the correct constraint,
    and format it in a way that mirrors the check exactly to
    make it clearer to people reading the code. The new error
    message reads "index must be larger than bus".
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1339900