qemu: setvcpus: Properly coldplug vcpus when hotpluggable vcpus are present
    
    When coldplugging vcpus to a VM that already has a few hotpluggable
    vcpus the code might generate invalid configuration as
    non-hotpluggable cpus need to be clustered starting from vcpu 0.
    
    This fix forces the added vcpus to be hotpluggable in such case.
    
    Fixes a corner case described in:
    https://bugzilla.redhat.com/show_bug.cgi?id=1370357