libxl: add a dummy ao_how to libxl_domain_core_dump

libxl: add a dummy ao_how to libxl_domain_core_dump

Although this function is not currently slow it may become so in the
future (this also depends somewhat on the size of the guest).
Therefore arrange for it to take an ao_how which it completes
immediately.  This will allow us to make it asynchronous in the future
without breaking API compatibility.

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