libxl: properly clean up array in libxl_list_cpupool failure path

libxl: properly clean up array in libxl_list_cpupool failure path

Document how cpupool_info works.  Distinguish success (ERROR_FAIL +
ENOENT) vs failure in libxl_list_cpupool and properly clean up the array
in failure path.

Also switch to libxl__realloc and call libxl_cpupool_{init,dispose}
where appropriate.

There is change of behaviour. Previously if memory allocation fails the
said function returns NULL. Now memory allocation failure is fatal. This
is in line with how we deal with memory allocation failure in other
places in libxl though.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>