tools/oxenstored: Automatically resume when possible

tools/oxenstored: Automatically resume when possible

When a `db` file exists use it to resume oxenstored.  It will contains a
xenstore tree, domain reconnection info, and watches.

It is currently missing data about all active socket connections, so a
toolstack should ideally be stopped and restarted too.

Tell systemd about oxenstored's PID and allow it to restart on success.

This should make updating oxenstored as easy as: `systemctl stop -s SIGTERM
xenstored` on a suitable xenstored version.

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>