intel10g.lua: Statistics registers are now all type "RC".

The copy & pasted definitions from the datasheet declared some as RO
or RW but in practice they seem to all have the behavior of clearing
after reading.
buffer.lua: Reset size when putting buffers on the free list.
virtio.lua: Run for 1 second instead of 1 minute.
Wrote loopback_test() and used it to debug intel10g.lua.