xen: disable interrupts early, as start_kernel expects

This avoids a lockdep warning from:
	if (DEBUG_LOCKS_WARN_ON(unlikely(!early_boot_irqs_enabled)))
		return;
in trace_hardirqs_on_caller();

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Xen-devel <xen-devel@lists.xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1672,6 +1672,9 @@ asmlinkage void __init xen_start_kernel(void)
 	   possible map and a non-dummy shared_info. */
 	per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];
 
+	local_irq_disable();
+	early_boot_irqs_off();
+
 	xen_raw_console_write("mapping kernel into physical memory\n");
 	pgd = xen_setup_kernel_pagetable(pgd, xen_start_info->nr_pages);