logging: introduce log handling protocol
    
    Define a new RPC protocol for the virtlogd daemon that provides
    for handling of logs. The initial RPC method defined allows a
    client to obtain a file handle to use for writing to a log
    file for a guest domain. The file handle passed back will not
    actually refer to the log file, but rather an anonymous pipe.
    The virtlogd daemon will forward I/O between them, ensuring
    file rotation happens when required.
    
    Initially the log setup is hardcoded to cap log files at
    128 KB, and keep 3 backups when rolling over, which gives
    a max usage of 512 KB per guest.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>