createhdds: use a temp file for virt-builder image creation
    
    Summary:
    Creating the virt-builder images directly with their final file
    name means that if we're rebuilding them for age and the build
    fails, we'll lose the existing image: it seems better to keep
    it than have no image at all, when this happens. It also means
    that while the rebuild is in process, the file might exist but
    be useless and cause any tests that happen to be running at the
    time to fail. So just like the guestfs images, create the file
    with a .tmp extension initially and rename it after a successful
    build.
    
    Test Plan:
    Do some image builds and check they work, check that
    temp file is cleaned up on failure and ctrl-c if you can...
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D858