Add more extensive tests

[cfergeau]

vfkit has some unit tests, but there are no tests starting a VM and checking the VM has the expected devices.
https://github.com/Code-Hex/vz makes use of https://github.com/Code-Hex/puipui-linux to start a very small VM and get a shell into the VM. vfkit could reuse this to implement various tests (check lspci output, test file sharing, test vsock listen and connect, ...) to catch regressions early.
https://github.com/cfergeau/vfkit/tree/puipui/ has code which downloads puipui, starts a VM and shuts it down through ssh.
We can build on that to add more useful tests.

[cfergeau]

This was done in #61
The tests for VM devices could be made even more extensive, but these will be good enough for now, let's close this.