tools/libxl: move libxenlight to tools/libs/light

Carve out all libxenlight related sources and move them to
tools/libs/light in order to use the generic library build environment.

The closely related sources for libxl-save-helper and the libxl test
environment are being moved, too.

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

tools/configure

@@ -585,7 +585,7 @@ PACKAGE_STRING='Xen Hypervisor Tools 4.15'
 PACKAGE_BUGREPORT='xen-devel@lists.xen.org'
 PACKAGE_URL='https://www.xen.org/'
 
-ac_unique_file="libxl/libxl.c"
+ac_unique_file="libs/light/libxl.c"
 # Factoring default headers for most tests.
 ac_includes_default="\
 #include <stdio.h>