xen: do not initialize PV timers on HVM if !xen_have_vector_callback

if !xen_have_vector_callback do not initialize PV timer unconditionally
because we still don't know how many cpus are available and if there is
more than one we won't be able to receive the timer interrupts on
cpu > 0.

This patch fixes an hang at boot when Xen does not support vector
callbacks and the guest has multiple vcpus.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: NJeremy Fitzhardinge <jeremy@goop.org>

arch/x86/xen/time.c

@@ -489,8 +489,9 @@ static void xen_hvm_setup_cpu_clockevents(void)
 __init void xen_hvm_init_time_ops(void)
 {
 	/* vector callback is needed otherwise we cannot receive interrupts
-	 * on cpu > 0 */
-	if (!xen_have_vector_callback && num_present_cpus() > 1)
+	 * on cpu > 0 and at this point we don't know how many cpus are
+	 * available */
+	if (!xen_have_vector_callback)
 		return;
 	if (!xen_feature(XENFEAT_hvm_safe_pvclock)) {
 		printk(KERN_INFO "Xen doesn't support pvclock on HVM,"