the facter is checking for /usr/sbin/xl and /usr/sbin/xm files do define the good commando to list the domains.
In some cases it is possible to have both files and be using xm. it will retunr an error.
$ sudo /usr/sbin/xl list
ERROR: A different toolstack (xm) have been selected!
If you use /usr/lib/xen-common/bin/xen-toolstack can catch the good command.