net/virtio: fix control VQ

net/virtio: fix control VQ

This patch mainly fixed below issues in the packed ring based
control vq support in virtio driver:

1. When parsing the used descriptors, we have to track the
   number of descs that we need to skip;
2. vq->vq_free_cnt was decreased twice for a same desc;

Meanwhile, make the function name consistent with other parts.

Fixes: ec194c2f1895 ("net/virtio: support packed queue in send command")
Fixes: a4270ea4ff79 ("net/virtio: check head desc with correct wrap counter")

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>