vhost: fix log base address

VHOST_SET_LOG_BASE got an incorrect address, causing
migration errors and potentially even memory corruption.

Reported-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Message-id: 1429283565-32265-1-git-send-email-mst@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>