libxl: ao abort: Support aborting where we spot domain death

libxl: ao abort: Support aborting where we spot domain death

Make an active libxl__domaindeathcheck contain an active
libxl__ao_abortable.

Consequential changes are:
 * domaindeath callbacks now take an rc value.
 * libxl__domaindeathcheck_start takes an ao, not a gc.
 * bootloader_domaindeath plumbs the rc through to its caller.
 * libxl__domaindeathcheck_init and _stop are not quite trivial any
   more so are moved from (inline functions) in libxl_internal.h, to
   ordinary functions defined in libxl_event.c.
 * libxl__domaindeathcheck_start is not trivial any more, and now has
   the standard error-handling pattern.

The only current user of libxl__domaindeathcheck is the bootloader.
So the result is that now it is possible to effectively abort domain
creation while the bootloader is running.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v2: New in this version of the series.