cli: Acquire and save kmsg kernel log during benchmark execution

We have had some issues with dodgy kernels falling over during benchmark
runs. But because we only have an SSH connection and don't have access
to the serial port, we can't see exactly what caused the panic.

Let's fix that by using netconsole to stream the kernel log over UDP to
the system running the fastpath driver. The log is then saved to a
per-session log file. If the machine panics we will see the panic log in
this file.

The file is named "session-<uuid>.kmsg" within the config directory.
Logging is best effort; it is fails to initialize we carry on.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>