vhost/vsock: refuse CID assigned to the guest->host transport

In a nested VM environment, we have to refuse to assign to a nested
guest the same CID assigned to our guest->host transport.
In this way, the user can use the local CID for loopback.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>