Virt-install's man page and virt-install --help both say to use the
following command to get the list of valid OS variant names:
osinfo-query os

But this command is provided by libosinfo-bin which is not a dependency
of virtinst and so was not installed on my system. I had to search on
internet to find the Debian package that provides it.
I believe it should be at least a recommended package.

Cesare.