windows: Check if VirtIO drivers are installed

If the virtio drivers are not installed on the media, raise a FatalError
Add an attribute in OSBase to host mount errors

Add the _mount_error attribute in OSBase. If mount fails, the error
message is written there.
Beautify the image creation output

 * Mount silently when mounting ro
 * Add a warning if no inspection method is available
 * In Windows put the "Preparing system for image creation" before
   mounting takes place
Minor fixes in the sysprep params menu

 * Use select_file() to update a "file" sysprep param
 * Switch the order of <Details> and <Update> buttons
 * Use the last visited menu entry as default item
Add a new "directory" type for sysprep params