xendomains init script; Add should-dependency on nfs-kernel-server

xendomains init script; Add should-dependency on nfs-kernel-server

Perhaps the guests use the hosts's NFS server, as in the case reported
in #826871.

In any case, it seems much more likely that on a system which is an
NFS server and also a Xen host, that the guests depend on the NFS,
rather than vice versa.

The obvious exception would be a driver domain.  Driver domains
probably need to be handled separately anyway, since they must start
before other domains.  (And anyway they are not particularly
well-handled by the current packaging.)

Closes: #826871
Signed-off-by: Ian Jackson <ian.jackson@citrix.com>