tools/xenstore: add live update command to xenstore-control

tools/xenstore: add live update command to xenstore-control

Add the "live-update" command to xenstore-control enabling updating
xenstored to a new version in a running Xen system.

With -c <arg> it is possible to pass a different command line to the
new instance of xenstored. This will replace the command line used
for the invocation of the just running xenstored instance.

The running xenstored (or xenstore-stubdom) needs to support live
updating, of course.

For now just add a small dummy handler to C xenstore denying any
live update action.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Wei Liu <wl@xen.org>