net/virtio: initiate vring at init stage

net/virtio: initiate vring at init stage

virtio_dev_vring_start() is actually doing the vring initiation job.
And the vring initiation job should be done at the dev init stage, as
stated with great details in former commit.

So move it there, and rename it to virtio_init_vring().

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>