vdpa_sim: advertise VIRTIO_NET_F_MTU

We've already reported maximum mtu via config space, so let's
advertise the feature.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-17-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>