runqemu: kill qemu if it hangs

qemu doesn't always behave well and can hang too.
kill it with force if it was still alive. Move clean up
commands into cleanup() function.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 079c2935)
Signed-off-by: Steve Sakoman <steve@sakoman.com>