store autoinst-log and last screenshot to testresults dir

This will redirect autoinst-log to be stored in testresults/<test-name> (Which is actualy a link to share/testresults/<test-name>).
New screenshot is copied from qemuscreenshots to testresults, the old one is removed, then last.png is relinked as before.  So that original change detection in OpenQA is still working.