mini-os/xenbus: Expose lower-level interface

mini-os/xenbus: Expose lower-level interface

Provide an interface that allows a xenbus user to explicitly allocate
ids, deal with responses asynchronously, specify the queues to be used
for responses and watches, etc.

More specifically:

* Enhance xenbus_event to be capable of dealing with both watches and
  command replies.  In particular, arrange that it will contain a
  pointer to the watch.  We leave the old fields undisturbed because
  of the way that this struct is already used in various places.

* Provide that a xenbus_event for a command response contains a copy
  of the pointer to the reply message, rather than putting it in the
  req_info (which is visible only internally).

* Rename `struct watch' to `struct xenbus_watch' because it needs
  to be in the public interface.

* allocate_xenbus_id becomes xenbus_id_allocate; same for release.

* Make xb_write into a public function, xenbus_xb_write.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>