VSOCK: Use callbacks for transport

Introduce a transport struct with callbacks, and switch to that from the direct
vmci_transport calls.  There's finally nothing left in af_vsock.c that is
vmci-specific, except the call to register with the transport (which is
topsy-turvy).

Acked-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Andy King <acking@vmware.com>