qemu code reading: bdrv_find_protocol() locate drv based on protocol
qemu code reading: detailed the relationship between BlockDriverState and BlockDriver
userspace code test: pcap example

Capture one packet.
userspace code test: capture packet in a loop
qemu code reading: cli option define and parse

In qemu, cli option part can be split into three parts:
1. option define
2. option register
3. option parse