vsock: add 'transport' member in the struct vsock_sock

As a preparation to support multiple transports, this patch adds
the 'transport' member at the 'struct vsock_sock'.
This new field is initialized during the creation in the
__vsock_create() function.

This patch also renames the global 'transport' pointer to
'transport_single', since for now we're only supporting a single
transport registered at run-time.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>