windows: Install viostor driver if missing

Add a dirty way to install the viostor driver in windows in order
to make them bootable. This works on Windows 2008R2
windows: Add REG_EXPAND_SZ() registry helper func

Add this helper lambda function that can be used to update the value of
type REG_EXPAND_SZ
windows: Add systemroot as a member in Windows

It is used many cases and it is better if we cache it.
windows: Upload the virtio drivers to the media

Upload the virtio drivers to the media and add the drivers directory to
the DevicePath Registry key