PCI: add xen dom0 checking before ACS initialization

This patch is predicated on Jeremy's patch in include/xen/xen.h.  It'll
prevent ACS init unless the platform has both an IOMMU and we're running
as dom0.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>