net/virtio-user: fix control VQ

net/virtio-user: fix control VQ

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

1. The idx_hdr should be used_idx instead of the id in the desc;
2. We just need to write out a single used descriptor for each
   descriptor list;
3. The avail/used bits should be initialized to 0;

Meanwhile, make the function name consistent with other parts.

Fixes: 48a4464029a7 ("net/virtio-user: support control VQ for packed")

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