tools/libxc: untangle libxenctrl from libxenguest

tools/libxc: untangle libxenctrl from libxenguest

Sources of libxenctrl and libxenguest are completely entangled. In
practice libxenguest is a user of libxenctrl, so don't let any source
libxenctrl include xg_private.h.

This can be achieved by moving all definitions used by libxenctrl from
xg_private.h to xc_private.h.

Export xenctrl_dom.h as it will now be included by other public
headers.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>