Add drv_to_virtio wrapper to get virtio_driver from device_driver.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: NWanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>