Update koan/virtinstall.py

'driver_type' replace with 'format'. makes it better work with image formats. with this replacement qcow2 works fine. 'driver_type' is an atavism in virt-install
Merge pull request #403 from jimi1283/master

Added check for wget/curl installation.
Merge pull request #393 from temujinZ/patch-2

Update koan/virtinstall.py
Changed F18 test to official release