Improve autodetection of productmd image type for osbuild images
    
    I don't love inferring the type from the filename like this -
    it's kinda backwards - but it's an improvement on the current
    logic (I don't think 'dvd' is ever currently the correct value
    here, I don't think osbuild *can* currently build the type of
    image that 'dvd' is meant to indicate). I can't immediately see
    any better source of data here (we could use the 'name' or
    'package_name' from 'build_info', but those are pretty much
    just inputs to the filenames anyway).
    
    Types that are possible in productmd but not covered here are
    'cd' (never likely to be used again in Fedora at least, not sure
    about RHEL), 'dvd-debuginfo' (again not used in Fedora, may be
    used in RHEL), 'ec2', 'kvm' (not sure about those), 'netinst'
    (this is a synonym for 'boot', we use 'boot' in practice in
    Fedora metadata), 'p2v' and 'rescue' (not sure).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>