block: Document that x-blockdev-change breaks quorum children list

Removing a quorum child node with x-blockdev-change results in a quorum
driver state that cannot be recreated with create options because it
would require a list with gaps. This causes trouble in at least
.bdrv_refresh_filename().

Document this problem so that we won't accidentally mark the command
stable without having addressed it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>

qapi/block-core.json

@@ -3918,6 +3918,10 @@
 # does not support all kinds of operations, all kinds of children, nor
 # all block drivers.
 #
+# FIXME Removing children from a quorum node means introducing gaps in the
+# child indices. This cannot be represented in the 'children' list of
+# BlockdevOptionsQuorum, as returned by .bdrv_refresh_filename().
+#
 # Warning: The data in a new quorum child MUST be consistent with that of
 # the rest of the array.
 #