x86/irq: optimize nr_irqs

Impact: make nr_irqs depend more on cards used in a system

depend on nr_irq_gsi more, and have a ratio for MSI.

v2: make nr_irqs less than NR_VECTORS * nr_cpu_ids
    aka if only one cpu, we only can support nr_irqs = NR_VECTORS

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>