Add test runner script qemu-xtf.sh which is allows any XTF x86 test to be
easily executed. Test runner is invoked from the qemu-smoke* jobs with the
hardcoded parameters.
Each x86 XTF job lead time is reduced a bit since only the test-related code
is built, not the entire XTF project.
Add .gitignore to avoid committing test artifacts by mistake.