createhdds: use user-mode networking for now
    
    There's been an annoying bug on the production openQA server
    for a while now: createhdds runs to refresh the upgrade base
    images don't actually work most of the time, each attempt to
    run virt-install fails. I finally got time to debug this a bit
    today and it seems to be some kind of network issue: the VM has
    no network access so the install doesn't work. I've no idea why
    that's happening, but using user-mode networking seems to work
    around it for now and shouldn't have any terrible side effects
    even for openQA servers not affected by this problem. So let's
    do this until we can pin down the real bug.