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As the current virtio-mmio only support single irq, so some advanced features such as vhost-net with irqfd are not supported. And the net performance is not the best without vhost-net and irqfd supporting. This patch support virtio-mmio to request multiple irqs like virtio-pci. With this patch and qemu assigning multiple irqs for virtio-mmio device, it's ok to use vhost-net with irqfd on