tools/oxenstored: Add cooperative live-update command

tools/oxenstored: Add cooperative live-update command

See docs/misc/xenstore.txt for documentation on live-update command.  Validate
that the binary exists and that the cmdline is valid, to prevent typos from
taking down xenstore; if live-update fails there is no way back due to the use
of exec().

Live update only proceeds if there are no active transactions, and no
unprocessed input or unflushed output.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Reviewed-by: Pau Ruiz Safont <pau.safont@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>