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/time.c b/arch/x86/xen/time.c
index 4780e55..2aab4a2 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -516,6 +516,7 @@
 	x86_platform.set_wallclock = xen_set_wallclock;
 }
 
+#ifdef CONFIG_XEN_PVHVM
 static void xen_hvm_setup_cpu_clockevents(void)
 {
 	int cpu = smp_processor_id();
@@ -544,4 +545,4 @@
 	x86_platform.get_wallclock = xen_get_wallclock;
 	x86_platform.set_wallclock = xen_set_wallclock;
 }
-
+#endif