Add vsock device support

[alexandruag]

This PR adds support for vsock devices, based on the backend and packet parser implementation currently available in Firecracker (some more details about the Firecracker Unix Domain Socket vsock backend are available here). The code is written and brought over on top of #49.

One way to verify the vsock device operation is to follow the next steps. First, build the vsock helper program from here (ideally as a static binary) and copy the binary to the guest filesystem image. After the guest boots with a vsock device added (i.e. by appending something like --vsock cid=5,uds_path=/tmp/vsock_uds0 to the vmm command line), start the listener server within the guest using ./vsock_helper echosrv 5252 (5252 is an arbitrary port number). Then, on the host, we can use something like the following script to verify that can send some data over the UDS vsock backend, and get a reply:

import re
from socket import socket, AF_UNIX, SOCK_STREAM

uds_path = '/tmp/vsock_uds0'
port = 5252

sock = socket(AF_UNIX, SOCK_STREAM)
sock.connect(uds_path)

buf = bytearray("CONNECT {}\n".format(port).encode("utf-8"))
assert sock.send(buf) == len(buf)

ack_buf = sock.recv(32)
assert re.match("^OK [0-9]+\n$", ack_buf.decode('utf-8')) is not None

msg = "vsock is working"
msg_buf = bytes(msg, 'utf-8')

sent = sock.send(msg_buf)
assert sent == len(msg_buf)

recv_buf = sock.recv(sent)
assert len(recv_buf) == len(msg_buf)
assert recv_buf == msg_buf

print(recv_buf.decode('utf-8'))

We're going to bring over parts of the vsock test infrastructure as well after advancing the discussion around dirty page tracking in vm-memory and seeing how the ongoing vm-virtio discussions conclude. The former is important because we want to start using VolatileSlices for the VsockPacket abstractions, while the latter influence the boiler plate code we have to adapt as part of the testing infrastructure.

[andreeaflorescu]

Closing this as we are going to add support once we upstream the components in vm-virtio.