Merge PR #8 (feature-windows-improvements) from skalkoto/snf-image-creator

Changes include:

 * Break windows.py into multiple modules.
 * Rewrite sysprep parameters mechanism and add a menu entry for this
   in snf-image-creator expert mode.
 * Detect if the media is already sysprepped before performing further
   customizatog.
 * Don't ask for windows password before customization (reset it if needed)
 * Detect if the virtio drivers are installed in the media.
   If not refuse to perform the customization.