New export_logs helper function

Exports .xsession-errors, systemd journal and /var/log/X* for further analysis of crashes
Use new export_logs helper function by first_boot and desktop_finish post_fail_hook
Merge pull request #190 from DimStar77/finish_desktop_post_hook

Export logs for finish_desktop, to be able to analyze some crashes
Merge pull request #195 from os-autoinst/desktop_change_for_sle11

Desktop change for sle11
Merge remote-tracking branch 'origin/master' into sle11