libxl: allow guest to write "control/shutdown" xenstore node.

libxl: allow guest to write "control/shutdown" xenstore node.

The PV shutdown/reboot/suspend protocol requires that the guest
acknowledge a request by clearing the node therefore it is necessary
to allow the guest to write to the node.

Currently libxl is quite relaxed about this protocol and doesn't
reeally seem to mind that the guest is unable to write the node to
perform the acknowledgement. However in a followup patch libxl needs
to be able to detect that a guest has acknowledged a suspend request.

A side effect of this change is that an empty "control/shutdown" node
is created upon domain creation instead of only being created when a
shutdown/reboot/suspend is requested. This should not (and does not
in my tests) have any negative impact on the guest.

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