virtio: make del_vqs idempotent

Our code calls del_vqs multiple times, assuming
it's idempotent.

commit 3ec7a77b
    virtio_pci: free up vq->priv
broke this assumption, by adding kfree there,
so multiple calls cause double free.

Fix it up.

Fixes: 3ec7a77b
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>