Introduce CONFIG_XEN_PVHVM compile option

This patch introduce a CONFIG_XEN_PVHVM compile time option to
enable/disable Xen PV on HVM support.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 84648c1..413b19b 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1942,6 +1942,7 @@
 	pv_mmu_ops = xen_mmu_ops;
 }
 
+#ifdef CONFIG_XEN_PVHVM
 static void xen_hvm_exit_mmap(struct mm_struct *mm)
 {
 	struct xen_hvm_pagetable_dying a;
@@ -1973,6 +1974,7 @@
 	if (is_pagetable_dying_supported())
 		pv_mmu_ops.exit_mmap = xen_hvm_exit_mmap;
 }
+#endif
 
 #ifdef CONFIG_XEN_DEBUG_FS