Add gvproxy benchmarks

[nirs]

vfkit and krunkit are integrated now only with gvproxy, which does not provide shared or bridged network and is 2-8 times slower.

• Add script to run gvproxy and vfkit/krunkit and run benchmarks
• Add gvproxy plot with vfkit and krunkit, comparing to vmnet-helper
• Add gvproxy/krunkit offloading plot

Flow:

• Start gvproxy with unix socket and api socket
• Wait until the api socket is created
• Set up port forwarding for iperf3 port on the host via the api socket
• Start vfkit, krunkit, krunkit with disabled offloading with the unix socket
• Wait for ip address in serial.log
• Run host-to-vm and vm-to-host benchmarks
• Store results in out/gvproxy/portfwd-vfkit-1/{host-to-vm,vm-to-host}.json
• terminate vfkit, gvproxy

Can be similar to https://github.com/lima-vm/socket_vmnet/blob/master/test/perf.sh