vsock/virtio: add SIOCOUTQ support for all virtio based transports

Introduce support for virtio_transport_unsent_bytes
ioctl for virtio_transport, vhost_vsock and vsock_loopback.

For all transports the unsent bytes counter is incremented
in virtio_transport_get_credit.

In virtio_transport (G2H) and in vhost-vsock (H2G) the counter
is decremented when the skbuff is consumed. In vsock_loopback the
same skbuff is passed from the transmitter to the receiver, so
the counter is decremented before queuing the skbuff to the
receiver.

Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>