qemu: don't assume slot 0 is unused/reserved.
    
    When qemuAssignDevicePCISlots() is looking for companion controllers
    for a USB controller that has no PCI address specified, it initializes
    a virDevicePCIAddress to 0000:00:00.0, fills it in with the
    companion's address if one is found, then checks whether or not there
    was a find based on slot == 0. On a system with a single PCI bus, that
    is a valid way to check, because slot 0 is reserved, but on most other
    PCI buses, slot 0 is not reserved, and is open for use by any
    device. This patch adds a separate bool that is set when a companion
    is found rather than relying on the faulty information provided with
    "slot == 0".