pds_vdpa: clear config callback when status goes to 0

If the client driver is setting status to 0, something is
getting shutdown and possibly removed.  Make sure we clear
the config_cb so that it doesn't end up crashing when
trying to call a bogus callback.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20231110221802.46841-3-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>