libxl: use LOG and LOGE instead of LIBXL__LOG* in libxl_utils.c

libxl: use LOG and LOGE instead of LIBXL__LOG* in libxl_utils.c

Code cleanup - no functional changes

The convenience macros LOG and LOGE have been written to take the
place of the old macros in the LIBXL__LOG* family. Replace the
invocations of the old macros in the function libxl_read_file_contents()
with invocations of the corresponding new ones. Create a local
libxl__gc gc* for the new macros to use by invoking GC_INIT(ctx) at the
top of the function, and clean it up by invoking GC_FREE at the two
exit points.

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