oeqa qemurunner.py: try to avoid reading one character at a time

Read from serial console with a small delay to bundle data to e.g.
full lines. Reading one character at a time is not needed and causes
busy looping.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
(cherry picked from commit 0049f675)
Signed-off-by: Steve Sakoman <steve@sakoman.com>