libxl: Add a gc to libxl_cpumap_alloc, ..._to_hex_string

libxl: Add a gc to libxl_cpumap_alloc, ..._to_hex_string

In the next patch we are going to change the definition of NOGC to
require a local variable libxl__gc *gc.  And this means that passing 0
to libxl__calloc is going to be wrong.

libxl_cpumap_alloc doesn't have a gc but passes 0 to libxl_calloc
Fix this by:
 - introducing an `out' label and an rc variable
 - replacing the returns with  rc = ERROR_BLAH; goto out;
 - adding uses of GC_INIT and GC_FREE.
 - changing NULL to NOGC in the call to libxl__calloc

Likewise fix libxl_cpumap_to_hex_string by:
 - adding a libxl_ctx* parameter and updating the one call site
 - adding uses of GC_INIT and GC_FREE.
 - changing NULL to NOGC in the call to libxl__zalloc

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Dario Faggioli <raistlin@linux.it>
Acked-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>