libxl: Use GC_INIT and GC_FREE everywhere

libxl: Use GC_INIT and GC_FREE everywhere

Replace
    libxl__gc gc = LIBXL_INIT_GC(ctx);
    ...
    libxl__free_all(&gc);
with
    GC_INIT(ctx);
    ...
    GC_FREE;
throughout with a couple of perl runes.

We must then adjust uses of the resulting gc for pointerness, which is
mostly just replacing all occurrences of "&gc" with "gc".  Also a
couple of unusual uses of LIBXL_INIT_GC needed to be fixed up by hand.

Here are those runes:
 perl -i -pe 's/\Q    libxl__gc gc = LIBXL_INIT_GC(ctx);/    GC_INIT(ctx);/' tools/libxl/*.c
 perl -i -pe 's/\Q    libxl__free_all(&gc);/    GC_FREE;/' tools/libxl/*.c

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