oeqa/qemurunner: Improve handling of run_serial for shutdown commands

When running a shutdown command, the serial port can close without the
command returning. This is seen as the socket being readable but having
no data. Change the way this case is handled in the code to avoid
tracebacks.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 396a3ba8)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>