Clean up handling of IS_PRIV_FOR() and rcu_[un]lock_domain().

Clean up handling of IS_PRIV_FOR() and rcu_[un]lock_domain().

In particular this *removes* some IS_PRIV_FOR() checks. *Especially*
in particular, all domctls are executable only by dom0. Several of
them were really unsafe for execution by a stub domain as they can
affect global system resource usage.

This probably breaks stub domains. Where necessary, some of these
reversions can themselves be reverted where they are judged both
necessary and safe.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>