qemu: Add check for PCI bridge placement if there are too many PCI devices
    
    Previous patch of this series fixed the issue with adding a new PCI bridge
    when all the slots were reserved by devices with user specified addresses.
    In case there are still some PCI devices waiting to get a slot reserved
    by qemuAssignDevicePCISlots, this means a new bus needs to be
    created along with a corresponding bridge controller. By adding an
    additional check, this scenario now results in a reasonable error
    instead of generating wrong qemu command line.