logging: change log protocol to be more reusable
    
    The current virtlogd RPC protocol provides the ability to
    handle log files associated with QEMU stdout/err. The log
    protocol messages take the virt driver, domain name and
    use that to form a log file path. This is quite restrictive
    as it prevents us re-using the same RPC protocol messages
    for logging to char device backends where the filename
    can be arbitrarily user specified. It is also bad because
    it means we have 2 separate locations which have to decide
    on logfile name.
    
    This change alters the RPC protocol so that we pass the
    desired log file path along when opening the log file
    initially. Now the virt driver is exclusively in charge
    of deciding the log filename
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>