libxl: provide progress reporting for long-running operations

libxl: provide progress reporting for long-running operations

This will be used for reporting, during domain creation, that the
console is ready.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Changes since v7:
 * If aop->how.callback, actually add the aop to the for_callback list (!)
 * Document the threadsafety of aop's, and make appropriate cross-references.
 * Allocate the actual aop from its thread's egc; do not free it.
 * Remove pointless code motion of libxl__ao_create.
 * Minor formatting fixes.
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>