vtd: add lookup_page method to iommu_ops

vtd: add lookup_page method to iommu_ops

This patch adds a new method to the VT-d IOMMU implementation to find the
MFN currently mapped by the specified DFN along with a wrapper function
in generic IOMMU code to call the implementation if it exists.

NOTE: This patch only adds a Xen-internal interface. This will be used by
      a subsequent patch.
      Another subsequent patch will add similar functionality for AMD
      IOMMUs.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>