oeqa/qemurunner: Improve logging thread exit handling for qemu shutdown test

Rather than totally disabling the logging, inform it we're about to exit
so we can log messages over the exit cleanly too. This aids debugging. It
also avoids a race where the logging handler could still error whilst
shutting down.

Also remove a race window by notificing the handler of the shutdown
first, before triggering it. This removes a race window I watched in
local testing.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>