add example to test vsock interface

[stlankes]

The example show the communication between host and HermitOS by a virtio socket device. See https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-39000010 for a detail description.

The example will be started by following command:

$ sudo qemu-system-x86_64 -smp 1 -cpu qemu64,apic,fsgsbase,rdtscp,xsave,xsaveopt,fxsr,rdrand -device isa-debug-exit,iobase=0xf4,iosize=0x04 -display none -m 1G -serial stdio -kernel hermit-loader-x86_64 -initrd target/x86_64-unknown-hermit/debug/vsock -device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=3

In this case, the Context IDentifier (CID) is defined as 3. The server is listening on port 9975 (= 0x0x26F7). On the host side, socat can be used to communicate with the server. The following command creates a connection to the server.

$ sudo socat - SOCKET-CONNECT:40:0:x00x00xF7x26x00x00x03x00x00x00x00x00x00x00

Note, 40 is Linux value for AF_VSOCK.

This PR depends on hermit-os/kernel#826