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

FooLinux:
    image_name = images/foo-linux

Which would make it possible to specify this custom guest using

$ avocado run migrate..tcp --vt-type qemu --vt-guest-os LinuxCustom.FooLinux

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

    # shell_prompt is a regexp used to match the prompt on aexpect.
    # if your custom os is based of some distro listed in the guest-os
    # dir, you can look on the files and just copy shell_prompt
    shell_prompt = "^\[.*\][\#\$]\s*$"
    # If you plan to use a raw device, set image_device = yes
    image_raw_device = yes
    # Password of your image
    password = 123456
    # Shell client used (may be telnet or ssh)
    shell_client = ssh
    # Port were the shell client is running
    shell_port = 22
    # File transfer client
    file_transfer_client = scp
    # File transfer port
    file_transfer_port = 22
