Draft 17

virtio-msg: fix-up response matching as discussed Oct 23

Simplify the response matching as discussed in the PR and in the Oct 23
meeting.

* Bus messages have a dev_num and it is normally 0 so we don't need to
  say that matching dev_num if only for transport messages.
* Drop msg_id from matching.  Altough the msg_id would normally match
  there may be some cases where it would not.  The token field should
  be unique for a given dev_num.
* This simplifys the description of the matching tuple as well

Signed-off-by: Bill Mills <bill.mills@linaro.org>