qemu: Allow multiple bridges when pci-bridges is not available
    
    qemuDomainAssignPCIAddresses() hardcoded the assumption
    that the only way to support devices on a non-zero bus is
    to add one or more pci-bridges; however, since we now
    support a large selection of PCI controllers that can be
    used instead, the assumption is no longer true.
    
    Moreover, this check was always redundant, because the
    only sensible time to check for the availability of
    pci-bridge is when building the QEMU command line, and
    such a check is of course already in place.
    
    In fact, there were *two* such checks, but since one of
    the two was relying on the incorrect assumption explained
    above, and it was redundant anyway, it has been dropped.