tools: switch XEN_LIBXEN* make variables to lower case (XEN_libxen*)

tools: switch XEN_LIBXEN* make variables to lower case (XEN_libxen*)

In order to harmonize names of library related make variables switch
XEN_LIBXEN* names to XEN_libxen*, as all other related variables (e.g.
CFLAGS_libxen*, SHDEPS_libxen*, ...) already use this pattern.

Rename XEN_LIBXC to XEN_libxenctrl, XEN_XENSTORE to XEN_libxenstore,
XEN_XENLIGHT to XEN_libxenlight, XEN_XLUTIL to XEN_libxlutil, and
XEN_LIBVCHAN to XEN_libxenvchan for the same reason.

Introduce XEN_libxenguest with the same value as XEN_libxenctrl.

No functional change.

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