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>