tools/hotplug: systemd xenstored dependencies

tools/hotplug: systemd xenstored dependencies

Everything which uses xenstored does this via the socket. Update the
existing service files to enable the xenstored.socket with the Requires=
statement. And schedule startup of the given service files after the
socket is enabled with the After= statement.
Once something tries to access the socket systemd will launch xenstored.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>