lguest: fix irq vectors.

	do_IRQ: cannot handle IRQ -1 vector 0x20 cpu 0
	------------[ cut here ]------------
	kernel BUG at arch/x86/kernel/irq_32.c:219!

We're not ISA: we have a 1:1 mapping from vectors to irqs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>