qmp: Use generic errors in memchar-read, memchar-write

New errors should be generic unless there's a real use case for rich
errors.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>