/etc/xdg/virt-builder/repos.d/libguestfs.conf is in libguestfs-tools 1:1.36.13-1ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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uri=http://libguestfs.org/download/builder/index.asc
gpgkey=file:///etc/xdg/virt-builder/repos.d/libguestfs.gpg
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