xen: fixes (authored by michalpalka)

Xen required a few changes in order to be usable:
* Include xenfs module in initrd as loading it in the activation
  script was failing.
* Include /etc/default/xendomains, which is needed by
  xen-domains service.
* Create /var/log/xen and /var/lib/xen directories in
  the xen-store service, which are needed by the xl command.
  The directories could be created by any other script as long as
  they are guaranteed to exist before xl is called.
* Fix a reference to /bin/ls in the xendomains script.