libxl: make LIBXL_INIT_GC a statement, not an initialiser

libxl: make LIBXL_INIT_GC a statement, not an initialiser

Previously LIBXL_INIT_GC was an initialiser, which you were expected
to use like this:
    libxl__gc gc = LIBXL_INIT_GC(ctx);

But we are going to want to put things in the gc which are to be
initialised using other macros.  That means that LIBXL_INIT_GC has to
become a statement too.  So instead, we make it so that it's used like this:
    libxl_gc gc;
    LIBXL_INIT_GC(gc,ctx);

In fact there are only a couple of callers now,
including GC_INIT which uses this trick:
    libxl_gc gc[1];
    LIBXL_INIT_GC(gc[0],ctx);

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>