x86: allow calling {shadow/hap}_set_allocation with the idle domain

x86: allow calling {shadow/hap}_set_allocation with the idle domain

... and using the "preempted" parameter. Introduce a new helper that can
be used from both hypercall or idle vcpu context (ie: during Dom0
creation) in order to check if preemption is needed. If such preemption
happens, the caller should then call process_pending_softirqs in order to
drain the pending softirqs, and then call *_set_allocation again to continue
with it's execution.

This allows us to call *_set_allocation() when building domain 0.

While there also document hypercall_preempt_check and add an assert to
local_events_need_delivery in order to be sure it's not called by the idle
domain, which doesn't receive any events (and that in turn
hypercall_preempt_check is also not called by the idle domain).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>