[virtio] Simplify virtqueue shutdown

[virtio] Simplify virtqueue shutdown

This commit introduces virtnet_free_virtqueues called on all virtqueue
error and shutdown paths. vpm_find_vqs no longer cleans up after itself
and instead expects virtnet_free_virtqueues to be always called to undo
its effect.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>