libxl: make libxl_device_pci_{add,remove,destroy} interfaces asynchronous

libxl: make libxl_device_pci_{add,remove,destroy} interfaces asynchronous

This does not make the implementation fully asynchronous but just
updates the API to support asynchrony in the future.

Currently although these functions do not call hotplug scripts etc and
therefore are not "slow" (per the comment about ao machinery in
libxl_internal.h) they do interact with the device model and so are
not quite "fast" either. We can live with this for now.

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