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>