libxl: provide TOSTRING in libxl_internal.h and libxlu_internal.h

libxl: provide TOSTRING in libxl_internal.h and libxlu_internal.h

Provide a copy of the standard TOSTRING macro in libxlu_internal.h,
for the benefit of patches later in this series.

Also, move TOSTRING to libxl_internal.h from a .c file for the
benefit of future other callers in libxl proper.

(These cannot be combined because libxlu cannot include
libxl_internal.h and libxl should not include libxlu_internal.h.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>