Revert 23733:fbf3768e5934 "AMD IOMMU: remove global ..."

Revert 23733:fbf3768e5934 "AMD IOMMU: remove global ..."

23733:fbf3768e5934 causes xen-unstable not to boot on several of the
xen.org AMD test systems.  We get an endless series of these:

  (XEN) AMD-Vi: IO_PAGE_FAULT: domain = 0, device id = 0x00a0, fault
  address = 0xfdf8f10144

I have constructed the attached patch which reverts c/s 23733
(adjusted for conflicts due to subsequent patches).  With this
reversion Xen once more boots on these machines.

23733 has been in the tree for some time now, causing this breakage,
and has already been fingered by the automatic bisector and discussed
on xen-devel as the cause of boot failures.  I think it is now time to
revert it pending a correct fix to the original problem.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>