libvirtqueue: driver_send uses word_size for len

Updated offset calculation of the 'message_len' variable to also
used word_size.

libvirtqueue: Added select_loop function

Added a select loop that loops over a set of virtqueues, calling
a parameterised function if there is data available.

libvirtqueue: Added libvirtqueue library

Added the cakeml virtqueue library. This has been ported from
the zmq camkes-vm-example. This also includes CMake helpers that
template cakeml libraries into a linear dependency chain.