block: don't register quorum driver if SHA256 support is unavailable

Commit 488981a4 [block: convert quorum blockdrv to use crypto APIs]
broke qemu-iotest 041 on hosts with GnuTLS < 2.10.0. It converted a
compile-time check to a run-time check at device open time. The result
is that we now advertise a feature (the quorum block driver) that will
never work (on those hosts). There's no way (short of parsing
human-readable error messages) for qemu-iotests or any other API
consumer to recognise that the quorum block driver isn't _actually_
available and shouldn't be used or tested.

Move the run-time check to bdrv_quorum_init() to avoid registering the
quorum block driver if we know it cannot work. This way API consumers
can recognise it's unavailable.

Fixes: 488981a4Signed-off-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Message-id: 1438699705-21761-1-git-send-email-silbe@linux.vnet.ibm.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

block/quorum.c

@@ -865,12 +865,6 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     int i;
     int ret = 0;
 
-    if (!qcrypto_hash_supports(QCRYPTO_HASH_ALG_SHA256)) {
-        error_setg(errp,
-                   "SHA256 hash support is required for quorum device");
-        return -EINVAL;
-    }
-
     qdict_flatten(options);
 
     /* count how many different children are present */
@@ -1061,6 +1055,10 @@ static BlockDriver bdrv_quorum = {
 
 static void bdrv_quorum_init(void)
 {
+    if (!qcrypto_hash_supports(QCRYPTO_HASH_ALG_SHA256)) {
+        /* SHA256 hash support is required for quorum device */
+        return;
+    }
     bdrv_register(&bdrv_quorum);
 }