qmp: Clean up design of memchar-read

The data returned has a well-defined size, which makes the size
returned along with it redundant at best.  Drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 8468f10..51ce2e6 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -525,7 +525,7 @@
                 "arguments": { "device": foo,
                                "size": 1000,
                                "format": "utf8" } }
-<- { "return": { "data": "data string...", "count": 1000 } }
+<- {"return": "abcdefgh"}
 
 EQMP