Do not allow HVM save/restore of a dying domain.

Do not allow HVM save/restore of a dying domain.

It's not a sensible thing to do, and it is easier than auditing all
state save/restore functions for safety (most importantly, racing
against domain_relinquish_resources).

Also place a spin_barrier on domain_lock after asserting d->is_dying,
allowing critical regions under the domain_lock to safely check
is_dying and avoid races with domain_relinquish_resources().

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