x86/mm: Make {hap, shadow}_teardown() preemptible

x86/mm: Make {hap, shadow}_teardown() preemptible

A domain with sufficient shadow allocation can cause a watchdog timeout
during domain destruction.  Expand the existing -ERESTART logic in
paging_teardown() to allow {hap/sh}_set_allocation() to become
restartable during the DOMCTL_destroydomain hypercall.

Signed-off-by: Anshul Makkar <anshul.makkar@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>