vdpa_sim: factor out buffer completion logic

Wrap up common buffer completion logic in to vdpasim_net_complete

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-18-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>