vl: Improve error message when we can't load fw_cfg from file

parse_fw_cfg() reports "can't load" without further details.  Get
the details from g_file_get_contents(), and include them in the
error message.
Signed-off-by: NLi Qiang <liq3ea@gmail.com>
Message-Id: <1541051971-28584-1-git-send-email-liq3ea@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

vl.c

@@ -2250,8 +2250,10 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
         size = strlen(str); /* NUL terminator NOT included in fw_cfg blob */
         buf = g_memdup(str, size);
     } else {
-        if (!g_file_get_contents(file, &buf, &size, NULL)) {
-            error_setg(errp, "can't load %s", file);
+        GError *err = NULL;
+        if (!g_file_get_contents(file, &buf, &size, &err)) {
+            error_setg(errp, "can't load %s: %s", file, err->message);
+            g_error_free(err);
             return -1;
         }
     }