virtio: add support for vsock

The "run" command accepts a new option (--vsock <cid>) which specify the
guest CID. For instance:

  $ lkvm run --kernel ./bzImage --disk test --vsock 3

One can easily test by: https://github.com/stefanha/nc-vsock.

In the guest:

  # modprobe vsock
  # nc-vsock -l 1234

In the host:

  # modprobe vhost_vsock
  # nc-vsock 3 1234

This patch comes from the early submission of G. Campana. On this basis,
I fixed the compilation errors and runtime crashes. Thanks for the work
done by G. Campana.
https://patchwork.kernel.org/patch/9542313/



Signed-off-by: G. Campana <gcampana+kvm@quarkslab.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20200915094402.107988-1-tianjia.zhang@linux.alibaba.com


Signed-off-by: Will Deacon <will@kernel.org>