libxl: define a per-domain logger.
    
    libxl doesn't provide a way to write one log for each domain. Thus
    we need to demux the messages. If our logger doesn't know to which
    domain to attribute a message, then it will write it to the default
    log file.
    
    Starting with Xen 4.9 (commit f9858025 and following), libxl will
    write the domain ID in an easy to grab manner. The logger introduced
    by this commit will use it to demux the libxl log messages.
    
    Thanks to the default log file, this logger will also work with older
    versions of Xen.