xen: Fix bug `do_IRQ: cannot handle IRQ -1 vector 0x6 cpu 1'

Following commit 9c3f2468d8339866d9ef6a25aae31a8909c6be0d, do_IRQ()
looks up the IRQ number in the per-cpu variable vector_irq.

This commit makes Xen initialise an identity vector_irq map for both X86_32 and X86_64.

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