In a User-Mode Linux session:
$ systemd-detect-virt
none
Although it is possible to reliably detect virtualization:
$ cat /proc/cpuinfo
processor : 0
vendor_id : User Mode Linux
model name : UML
mode : skas
host : Linux kytes 3.11.0-rc1-00009-ge5fd680 (...)
bogomips : 7007.43
So, grep for the string "\nvendor_id\t: User Mode Linux\n" in
/proc/cpuinfo, and say "uml" when asked.
f33d3ec1 (move more common files to shared/ and add them to shared.la,
2012-04-12) moved src/hostname-setup.c to src/shared/hostname-setup.c,
but did not update DISTRO_PORTING accordingly. Fix this.