vhost: add vsock compat ioctl

This will allow usage of vsock from 32-bit binaries on a 64-bit
kernel.

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>