vhost: fix miration during device start

We need to know ring layout to allocate log buffer.
So init rings first.

Also fixes a theoretical memory-leak-on-error.

https://bugzilla.redhat.com/show_bug.cgi?id=615228

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
