virtio: Introduce virtqueue_drop_all procedure

Add procedure for fast drop of queued packets, acting like
pop and push without mapping the buffers into memory.

Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>