Hmm not sure why it didn't work when running the service. But if it is working now please mark the thread as solved
hugleo,Make your changes in the 09_xen config (XEN_HYPERVISOR_CMDLINE and XEN_LINUX_CMDLINE) as this is the file provided by the xen package. However looking at the PKGBUILD of the xen package it doesnt look like it does anything with the 09_xen file when an update happens. You will either need to add it yourself (to the backups or whatever) or ask the maintainer to update the pkgbuild.