PoD: appropriate BUG_ON when domain is dying

PoD: appropriate BUG_ON when domain is dying

BUG_ON(d->is_dying) in p2m_pod_cache_add() which is introduced in
c/s 20426 is not proper. Since dom->is_dying is set asynchronously.
For example, MMU_UPDATE hypercalls from qemu and the
DOMCTL_destroydomain hypercall from xend can be issued simultaneously.

Also this patch lets p2m_pod_empty_cache() wait by spin_barrier
until another PoD operation ceases.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>