libxl: wait for the ack when issuing power control requests

libxl: wait for the ack when issuing power control requests

Currently only suspend power control requests wait for an ack from the
domain, while power off or reboot requests simply write the command to
xenstore and exit.

Introduce a 1 minute wait for the domain to acknowledge the request, or
else return an error. The suspend code is slightly modified to use the
new infrastructure added, but shouldn't have any functional change.

Fix the ocaml bindings and also provide a backwards compatible
interface for the reboot and poweroff libxl API functions.

Reported-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
[ wei: change ret to rc to fix build ]
Signed-off-by: Wei Liu <wl@xen.org>