hw/net/virtio-net: make some VirtIONet const

hw/net/virtio-net: make some VirtIONet const

The VirtIONet structure is not modified in
virtio_net_supported_guest_offloads().
Therefore, make it const to allow this function to
accept const variables.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <489b09c3998ac09b9135e57a7dd8c56a4be8cdf9.1685704856.git.yin31149@gmail.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Tested-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>