scripts/qemutestlib: Add better process debugging and fix process group issue

In single testing with a shutdown scenario the processes are cleaned up correctly
but the manual cleanup fall back used for a minimal image do not work properly.
This patch fixes the kill commands to revert to non-process groups, fixing
the hung process issues that were occuring.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>