[installer][host] reenable IP validation
[installer][host] fix preseeding and smaller bugfixes related to OSX u/mount
[installer][host] fix error handling on image-write fail. At least for OSX
[installer][host] fix signatures for linux
[installer][host] fix for linux io method call
Merge pull request #30 from srmo/master

Fix the Preseeding (at least on OSX) and additional, minor and major fixes