port.lua: Updated spam() function to work again.
virtio.lua: Optimize kick() to use VRING_F_NO_NOTIFY.

That is: don't kick the kernel's eventfd if it advertises that this
isn't needed. This should mean that we only kick the eventfd when the
kernel doesn't know there is any data available for processing i.e.
coalesce kicks during bursts of transfers.
virtio.lua: Test with spam() instead of echo().

That way something interesting happens without requiring you to
manually run processes on the Linux side e.g. ifconfig/arp/ping/etc.
virtio_vring.h: Added VRING_F_NO_NOTIFY constant.