lib: guestfs_disk_format: Set LC_ALL=C instead of LANG=C.

LC_ALL takes precedence over LANG.  See:

http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html

src/info.c

@@ -190,7 +190,7 @@ run_qemu_img_info (guestfs_h *g, const char *filename,
     dup2 (fd[1], 1);
     close (fd[1]);
 
-    setenv ("LANG", "C", 1);
+    setenv ("LC_ALL", "C", 1);
 
     /* XXX stderr to event log */