x86, VisWS: turn into generic arch, add NR_IRQS quirk

NR_IRQS: let VISWS be just a sub-case of the generic code.

This can create a somewhat larger irq_desc[] array if NR_CPUS is high
but that should not worry VisWS which has 4 CPUs at most.

Signed-off-by: Ingo Molnar <mingo@elte.hu>