Rewrite the sysprep parameter mechanism

Rewrite the mechanism to have better control on the system preparation
parameters. Also change the way the parameters are updated in the UI.
Rewrite file selection function

Make it more general. Till know it was only used for selecting the input
media but other parts of the program should also make use of it.
Check for Winexe when customizing

We used to check for the presence of the winexe executable in the
Windows __init__ function. This is not optimal. The winexe check
should be done during customization. The user should be able to
dump, upload or register an image without having Winexe installed.
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