qemu: driver: Validate configuration when setting maximum vcpu count
    
    Setting vcpu count when cpu topology is specified may result into an
    invalid configuration. Since the topology can't be modified, reject the
    setting if it doesn't match the requested topology. This will allow
    fixing the topology in case it was broken.
    
    Partially fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1370066