logging: preserve driver, dom name & uuid against log file
    
    The virt driver, dom name and uuid associated with a log
    file are important pieces of metadata to keep around for
    sake of future enhancements to virtlogd. Currently we
    discard them after opening the log file, but we should
    preserve them, even across restarts.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>