libxl: Make all hidden/static functions take a gc not a ctx

libxl: Make all hidden/static functions take a gc not a ctx

Also ensure that static and hidden functions use the libxl__ prefix
not just libxl_ (in the case of static functions only when they use a
libxl prefix to start with).

This follows the policy described in libxl.h "libxl memory
management".

Based on a manual audit of:
grep ^static tools/libxl/libxl*.[ch]| grep libxl_ctx
grep libxl__ tools/libxl/*.h| grep libxl_ctx

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>