rotatingfile: mark log files as close-on-exec
    
    The log file descriptor associated with the virRotatingFile
    struct should be marked close-on-exec, as even when virtlogd
    re-exec's itself it expect to open the log file fresh. It
    does not need to preserve the logfile handles, only the network
    client FDs.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>