qemurunner: Improve handling of serial port output blocking

qemurunner: Improve handling of serial port output blocking

Similar to stdout in the previous commit, we need to ensure serial output
if written is read and put somewhere, else qemu might block on writes to
the serial port leading to hangs in the kernel. Use our existing logging
thread to log data when run_serial is not in use.

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