maintainers: add vhost and virtio co-maintainers

maintainers: add vhost and virtio co-maintainers

Add Tiwei and Zhihong as co-maintainers for the Vhost and
Virtio components. They have done great contributions recently,
and been very helpful in helping to review Vhost and Virtio series.

Also, add Tiwei as backup for the Next-virtio tree.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Tiwei Bie <tiwei.bie@intel.com>
Acked-by: Zhihong Wang <zhihong.wang@intel.com>
Acked-by: Zhiyong Yang <zhiyong.yang@intel.com>