qemurunner: support serial console login via qemu stdout

runqemu script works with qemu machines which provide login
and serial console to the qemu process stdout. Add the same support
to qemurunner so that testing with testimage.bbclass is possible.

Default qemu machines provide serial console boot logs and login via
socket to qemu process but I don't see a reason why qemu process stdout
should not be supported too since they work with runqemu as well.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>