crypto: add "none" random provider

In case of not using random-number needing feature, it makes sense to
skip RNG init too. This is especially helpful when QEMU is sandboxed in
Stubdomain under Xen, where there is very little entropy so initial
getrandom() call delays the startup several seconds. In that setup, no
random bytes are needed at all.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

crypto/random-none.c

@@ -0,0 +1,38 @@
+/*
+ * QEMU Crypto "none" random number provider
+ *
+ * Copyright (c) 2020 Marek Marczykowski-Górecki
+ *                      <marmarek@invisiblethingslab.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "qemu/osdep.h"
+
+#include "crypto/random.h"
+#include "qapi/error.h"
+
+int qcrypto_random_init(Error **errp)
+{
+    return 0;
+}
+
+int qcrypto_random_bytes(void *buf,
+                         size_t buflen,
+                         Error **errp)
+{
+    error_setg(errp, "Random bytes not available with \"none\" rng");
+    return -1;
+}