block: Clean up "Could not create temporary overlay" error message

bdrv_create() sets an error and returns -errno on failure.  When the
latter is interesting, the error is created with error_setg_errno().

bdrv_append_temp_snapshot() uses the error's message to create a new
one with error_setg_errno().  This adds a strerror() that is either
uninteresting or duplicate.  Use error_setg() instead.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-7-git-send-email-armbru@redhat.com>

block.c

@@ -1463,9 +1463,8 @@ int bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp)
     ret = bdrv_create(&bdrv_qcow2, tmp_filename, opts, &local_err);
     qemu_opts_del(opts);
     if (ret < 0) {
-        error_setg_errno(errp, -ret, "Could not create temporary overlay "
-                         "'%s': %s", tmp_filename,
-                         error_get_pretty(local_err));
+        error_setg(errp, "Could not create temporary overlay '%s': %s",
+                   tmp_filename, error_get_pretty(local_err));
         error_free(local_err);
         goto out;
     }