x86/mm/paging: Prevent the guest from faulting on the same gfn when dying

x86/mm/paging: Prevent the guest from faulting on the same gfn when dying

A crashing guest due to paging may hit an "endless" loop faulting repeatedly on
the current paged-out gfn, until the toolstack comes around to killing the
domain.

Unfortunately domain_crash at this point may not pause the vcpu due to the
shutting_down flag, so we pause it explicitly when detecting the condition.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>