tools/xendomains: move to libexec and use a smaller init helper

tools/xendomains: move to libexec and use a smaller init helper

The xendomains script can be reused with systemd systems as it
does not control services or sockets per se, but does a one shot
scrape of domUs it needs start bring up, stop, reload so we're
going to reuse it. This moves the core of the script to libexec
and leaves only a helper on init.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Jan Rękorajski <baggins@pld-linux.org>
Cc: M A Young <m.a.young@durham.ac.uk>
Cc: Jacek Konieczny <jajcus@jajcus.net>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>