VSOCK: transport-specific vsock_transport functions

struct vsock_transport contains function pointers called by AF_VSOCK
core code.  The transport may want its own transport-specific function
pointers and they can be added after struct vsock_transport.

Allow the transport to fetch vsock_transport.  It can downcast it to
access transport-specific function pointers.

The virtio transport will use this.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>