Support Desktop variants of Microsoft Windows

Update the unattend.xml file to support Desktop variants of Microsoft
Windows.
docs: Add a Windows deployment section

This contains basic info on how the Windows images get deployed and
instruction on how to use a custom Unattend.xml file.
windows: Consider Autounattend.xml as answer file

When searching a windows image for the presence of an answer file,
make sure that Autounattend.xml is also considered as answer files.
docs: Fix an error about the img_personality field

Fix the documentation to state that the mode field should always
contain a decimal number.