libxl: Asynchronous/long-running operation infrastructure

libxl: Asynchronous/long-running operation infrastructure

Provide a new set of machinery for writing public libxl functions
which may take a long time.  The application gets to decide whether
they want the function to be synchronous, or whether they'd prefer to
get a callback, or an event, when the operation is complete.

User(s) of this machinery will be introduced in later patch(es).

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>