libxl: use LOG instead of LIBXL__LOG in libxl_utils.c

libxl: use LOG instead of LIBXL__LOG in libxl_utils.c

To conform to the new coding style, replace the invocation of
LIBXL__LOG in the function libxl_pipe() in the file libxl_utils.c
with an invocation of LOG. Create a local libxl__gc gc* for LOG
to use by invoking GC_INIT(ctx) at the top of the function, and
clean it up by invoking GC_FREE at the exit. Create a variable,
ret, to consolidate exits in one place and avoid invoking GC_FREE
twice.

Suggested-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Kelley Nielsen <kelleynnn@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>