You can drop files in this dir to define custom windows guests.
As an example, you could create a file called foo.cfg with the contents:

FooWindows:
    image_name = images/foo-windows

Which would make it possible to specify this custom guest using

$ avocado run migrate..tcp --vt-type qemu --vt-guest-os WindowsCustom.FooWindows

Other useful params to set (not an exaustive list):

    # If you plan to use a raw device, set image_device = yes
    image_raw_device = yes
    # Attention: Changing the password in this file is not supported,
    # since files in winutils.iso use it.
    username = Administrator
    password = 1q2w3eP

