tools: split libxenstat into new tools/libs/stat directory

tools: split libxenstat into new tools/libs/stat directory

There is no reason why libxenstat is not placed in the tools/libs
directory.

At the same time move xenstat.h to a dedicated include directory
in tools/libs/stat in order to follow the same pattern as the other
libraries in tools/libs.

As now xentop is the only left directory in xenstat move it directly
under tools and get rid of tools/xenstat.

Fix some missing prototype errors (add one prototype and make two
functions static).

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