info: disk-format et al: Fix error message when no file exists (RHBZ#981663).

$ guestfish disk-format /etc/nosuchfile

Instead of this obscure double error message:

  libguestfs: error: qemu-img info: JSON parse error: parse error: premature EOF
                       (right here) ------^
  libguestfs: error: qemu-img info: /etc/nosuchfile: qemu-img info exited with error status 1.
  To see full error messages you may need to enable debugging.
  See http://libguestfs.org/guestfs-faq.1.html#debugging-libguestfs

You will now see this simpler error:

  libguestfs: error: disk info: /etc/nosuchfile: No such file or directory

Thanks: Bo Fan.

src/info.c

@@ -26,6 +26,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/types.h>
+#include <sys/stat.h>
 #include <sys/wait.h>
 #include <assert.h>
 
@@ -235,17 +236,40 @@ static yajl_val
 get_json_output (guestfs_h *g, const char *filename)
 {
   CLEANUP_CMD_CLOSE struct command *cmd = guestfs___new_command (g);
-  int r;
+  int fd, r;
+  char fdpath[64];
   yajl_val tree = NULL;
+  struct stat statbuf;
+
+  fd = open (filename, O_RDONLY /* NB: !O_CLOEXEC */);
+  if (fd == -1) {
+    perrorf (g, "disk info: %s", filename);
+    return NULL;
+  }
+
+  if (fstat (fd, &statbuf) == -1) {
+    perrorf (g, "disk info: fstat: %s", filename);
+    close (fd);
+    return NULL;
+  }
+  if (S_ISDIR (statbuf.st_mode)) {
+    error (g, "disk info: %s is a directory", filename);
+    close (fd);
+    return NULL;
+  }
+
+  snprintf (fdpath, sizeof fdpath, "/dev/fd/%d", fd);
+  guestfs___cmd_clear_close_files (cmd);
 
   guestfs___cmd_add_arg (cmd, "qemu-img");
   guestfs___cmd_add_arg (cmd, "info");
   guestfs___cmd_add_arg (cmd, "--output");
   guestfs___cmd_add_arg (cmd, "json");
-  guestfs___cmd_add_arg (cmd, filename);
+  guestfs___cmd_add_arg (cmd, fdpath);
   guestfs___cmd_set_stdout_callback (cmd, parse_json, &tree,
                                      CMD_STDOUT_FLAG_WHOLE_BUFFER);
   r = guestfs___cmd_run (cmd);
+  close (fd);
   if (r == -1)
     return NULL;
   if (!WIFEXITED (r) || WEXITSTATUS (r) != 0) {