vsock: lookup and setup guest_cid inside vhost_vsock_lock

Multi vsocks may setup the same cid at the same time.

Signed-off-by: Gao feng <omarapazanadi@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>