x86/mm: make {set,clear}_identity_p2m_mapping() work for PV guests

Namely Dom0 suffers from commit 5ae03990c1 ("xen/vtd: create RMRR
mapping") having removed the creation of such mappings for non-
translated guests.

Reported-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -957,7 +957,11 @@ int set_identity_p2m_entry(struct domain
     int ret;
 
     if ( !paging_mode_translate(p2m->domain) )
-        return 0;
+    {
+        if ( !need_iommu(d) )
+            return 0;
+        return iommu_map_page(d, gfn, gfn, IOMMUF_readable|IOMMUF_writable);
+    }
 
     gfn_lock(p2m, gfn, 0);
 
@@ -1028,7 +1032,11 @@ int clear_identity_p2m_entry(struct doma
     int ret;
 
     if ( !paging_mode_translate(d) )
-        return 0;
+    {
+        if ( !need_iommu(d) )
+            return 0;
+        return iommu_unmap_page(d, gfn);
+    }
 
     gfn_lock(p2m, gfn, 0);