libxl: convert libxl__sprintf(gc) to GCSPRINTF

libxl: convert libxl__sprintf(gc) to GCSPRINTF

The rune used is:

  sed -i 's/libxl__sprintf(gc,\s*/GCSPRINTF(/g' libxl*.c

This rune is simple and better than trying to match every possible
patterns.

Two instances in libxl_dm.c need fixing up. They are in fact better to just
use libxl__strdup.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>