cxenstored: add support for systemd active sockets

cxenstored: add support for systemd active sockets

This adds systemd socket activation support for the C xenstored.
Active sockets enable xenstored to be loaded only if required by a system
onto which Xen is installed on. Socket activation is handled by
systemd, once a port for a service which claims a socket is used
systemd will start the required services for it, on demand. For more
details on socket activation refer to Lennart's socket-activation
post regarding this [0].

Right now this code adds a no-op for this functionality, leaving the
enablement to be done later once systemd is properly hooked into
the build system. The socket activation is ordered in aligment with
the socket activation order passed on to systemd.

[0] http://0pointer.de/blog/projects/socket-activation2.html

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>