qemurunner: Fix a bug with fork/exit handling

qemurunner: Fix a bug with fork/exit handling

If you send this forked process a SIGTERM, it will execute all of the
parent's exit code leading to two sets of console/exit output which is
extremely confusing. Wrap the code in a try/finally to ensure we always
call os._exit() to avoid this.

I spent far too long trying to work out the crazy console output from this.

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