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/enlighten.c b/arch/x86/xen/enlighten.c
index 75b479a..6f53453 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1282,6 +1282,7 @@
 	}
 }
 
+#ifdef CONFIG_XEN_PVHVM
 static int __cpuinit xen_hvm_cpu_notify(struct notifier_block *self,
 				    unsigned long action, void *hcpu)
 {
@@ -1338,3 +1339,4 @@
 	.init_platform		= xen_hvm_guest_init,
 };
 EXPORT_SYMBOL(x86_hyper_xen_hvm);
+#endif