Integration tests for vsocks

[xibz]

After discussion in #903, this issue was created to track adding integration testing of vsock.

[andreeaflorescu]

I just realized that all our integration tests are running firecracker jailed, so we will need to fix #911 and #897 in one go in order to be able to add integration tests. This was a design choice that we made some time ago as we want to test Firecracker as close as possible to a production setup (which should include the jailer).