qemu-testlib: Add python helper and simplify shell

The current code has a race since it greps for *any* qemu process
running, even if it isn't the one we started. This leads to some sanity
tests potentially failing on machines where multiple sets of sanity tests
are running.

To resovle this and some other ugly code issues, add a python script
to accurately walk the process tree and find the qemu process. We can
then replace all the shell functions attempting this which happen to
work in many cases but not all.

Also clean up some of the error handling so its more legible.

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