libfs,xenfs: replace /proc/xen/xenbus with a symlink

[David Vrabel]

Using /proc/xen/xenbus can cause deadlocks on the atomic file position
mutex since this file should behave like a character device and not a
regular file.  This is easiest to achive by making it a symlink to the
existing /dev/xen/xenbus device.

This requires extending simple_fill_super() to add symlinks as well as
regular files.

David