Fix insserv dependencies

(import of a Debian patch)

This patch removes xend from the list of dependencies.

Ganeti doesn't need xend running to startup, it will only need it later
(and only if xen is used as virtualisation technology). It also removes
'Xen' from the description in the init script.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>