x86: fix probe_nr_irqs for xen

otherwise Xen is _completely_ unusable with 5 or more VCPUs.
(when !CONFIG_HAVE_SPARSE_IRQ).

based on Alex Nixon's patch.

also add +1 offset after redir_entries

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Alex Nixon <alex.nixon@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>