qemu-char: Drop undocumented chardev "memory" compatibility syntax

This is a new device, so there's no compatibility to maintain, and its
use case isn't common enough to justify shorthand syntax.

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