libxl: ao abort: Provide public ao abort request API

libxl: ao abort: Provide public ao abort request API

Provide libxl_ao_abort.

There is machinery to allow an ao to register an interest in abort
requests, using a libxl__ao_abortable.

This API is not currently very functional: requesting abort will
never have any effect.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
---
v5: Clarify API doc comment re aborting synchronous operations.
v4: Rename from cancel to abort.
    Actually record aos on aos_inprogress.
    (Report from Koushik Chakravarty at Citrix.)
    Do not mark libxl_ao_cancel hidden (!)
    Abolish ERROR_NOTIMPLEMENTED from libxl_ao_cancel.
    All operations are supposed to support cancellation.
v2: Minor comment improvements