iommu: rename wrapper functions

iommu: rename wrapper functions

A subsequent patch will add semantically different versions of
iommu_map/unmap() so, in advance of that change, this patch renames the
existing functions to iommu_legacy_map/unmap() and modifies all call-sites.
It also adjusts a comment that refers to iommu_map_page(), which was re-
named by a previous patch.

This patch is purely cosmetic. No functional change.

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