qapi: Rename ChardevBackend member "memory" to "ringbuf"

qapi: Rename ChardevBackend member "memory" to "ringbuf"

Commit 1da48c6 called the new member "memory" after commit 3949e59
standardized "ringbuf".  Rename for consistency.

However, member name "memory" is visible in QMP since 1.5.  It's
undocumented just like the driver name.  Keep it working anyway.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1374849874-25531-4-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>