qemu-char: Register ring buffer driver with correct name "ringbuf"

qemu-char: Register ring buffer driver with correct name "ringbuf"

The driver is new in 1.4, with the documented name "ringbuf".
However, it's actual name is the completely undocumented "memory".
Screwed up in commit 3949e59.  Fix code to match documentation.

Keep the undocumented name working as an alias for compatibility.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1374849874-25531-3-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>