qemu: fix detach of hostdev based network interface

qemu: fix detach of hostdev based network interface

This fixes bug in

  commit bbe2aa627f621e6749af374b22856184d1f351dc
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Thu Jul 26 17:24:30 2018 +0100

    conf: simplify link from hostdev back to network device

    hostdevs have a link back to the original network device. This is fairly
    generic accepting any type of device, however, we don't intend to make
    use of this approach in future. It can thus be specialized to network
    devices.

    Reviewed-by: Cole Robinson <crobinso@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
which mistakenly deleted the assignment to the 'net' variable,
which meant we never invoked the network driver release callback

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>