x86: fix map_domain_page() leak from vcpu_destroy_pagetables()

Introduced by c/s 26450:4816763549e0 and exposed with
26523:fd997a96d448.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1997,6 +1997,7 @@ static void vcpu_destroy_pagetables(stru
         }
 
         l4e_write(l4tab, l4e_empty());
+        unmap_domain_page(l4tab);
 
         v->arch.cr3 = 0;
         return;