vsock/virtio_transport_common: remove unused virtio header includes

We can remove virtio header includes, because virtio_transport_common
doesn't use virtio API, but provides common functions to interface
virtio/vhost transports with the af_vsock core, and to handle
the protocol.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>