twister: Capture qemu launch stdout and stderr

Currently we launch qemu (well, "ninja run" usually) using Popen and
request stdout and stderr to be redirected into a pipe. However we never
read that pipe so the information is not captured.

Instead log directly into files that can be inspected after a failed
to to find out why qemu run failed.

Note that this is really only useful in cases where qemu either fails to
launch or crashes.

Regular test data is still handled via the qemu fifo.

Signed-off-by: Juha Kuikka <juha.kuikka@gmail.com>