base-files: add xenfs entry to fstab if xen DISTRO_FEATURE is enabled

Xen handles mounting xenfs with an init script. Unfortunately this
script is the same one that starts xenstored and xenconsoled. That's
great for dom0 but a domU may need xenfs but running xenstored/
xenconsoled in a domU makes no sense. A simple fstab entry is
preferrable.