xen/pvh: Use Xen's emergency_restart op for PVH guests

Using native_machine_emergency_restart (called during reboot) will
lead PVH guests to machine_real_restart()  where we try to use
real_mode_header which is not initialized.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/enlighten.c

@@ -1924,6 +1924,9 @@ static void __init xen_hvm_guest_init(void)
 	x86_init.irqs.intr_init = xen_init_IRQ;
 	xen_hvm_init_time_ops();
 	xen_hvm_init_mmu_ops();
+
+	if (xen_pvh_domain())
+		machine_ops.emergency_restart = xen_emergency_restart;
 #ifdef CONFIG_KEXEC_CORE
 	machine_ops.shutdown = xen_hvm_shutdown;
 	machine_ops.crash_shutdown = xen_hvm_crash_shutdown;