quorum: Return an error if the blkverify mode has invalid settings

The blkverify mode of Quorum can only be enabled if the number of
children is exactly two and the value of vote-threshold is also two.

If the user tries to enable it but the other settings are incorrect
then QEMU simply prints an error message to stderr and carries on
disabling the blkverify setting.

This patch makes quorum_open() fail and return an error in this case.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/quorum.c

@@ -912,13 +912,12 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     s->read_pattern = ret;
 
     if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
-        /* is the driver in blkverify mode */
-        if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
-            s->num_children == 2 && s->threshold == 2) {
-            s->is_blkverify = true;
-        } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
-            fprintf(stderr, "blkverify mode is set by setting blkverify=on "
-                    "and using two files with vote_threshold=2\n");
+        s->is_blkverify = qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false);
+        if (s->is_blkverify && (s->num_children != 2 || s->threshold != 2)) {
+            error_setg(&local_err, "blkverify=on can only be set if there are "
+                       "exactly two files and vote-threshold is 2");
+            ret = -EINVAL;
+            goto exit;
         }
 
         s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE,