vdpa: Add reset callback in vdpa_config_ops

This adds a new callback to support device specific reset
behavior. The vdpa bus driver will call the reset function
instead of setting status to zero during resetting.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210831103634.33-6-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>