chardev: Make the name of memory device consistent

chardev: Make the name of memory device consistent

Now we have memory char device, but the backend name of it
is a little confusion. We actually register it by 'memory', but
the description in qemu-option, the name of open functions
and the new api backend called it 'ringbuf'. It should keep
consistent. This patch named it all to 'memory'.

Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1369132079-11377-2-git-send-email-lilei@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>