vhost-user-blk: start vhost when guest kicks

vhost-user-blk: start vhost when guest kicks

Some old guests (before commit 7a11370e5: "virtio_blk: enable VQs early")
kick virtqueue before setting VIRTIO_CONFIG_S_DRIVER_OK. This violates
the virtio spec. But virtio 1.0 transitional devices support this behaviour.
So we should start vhost when guest kicks in this case.

Signed-off-by: Yongji Xie <[email protected]>
Signed-off-by: Chai Wen <[email protected]>
Signed-off-by: Ni Xun <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>