selftests/bpf: add vsock to vmtest.sh

Add vsock loopback to the test kernel.

This allows sockmap for vsock to be tested.

Signed-off-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>